I18n: fix translation of Grease Pencil 3.0 layer names

The Grease Pencil 3.0 layers and groups were not properly translated:
- The layers and groups' names were translated during display in the
  layer list. Since they are data, they should be left untranslated at
  this time.
- On object creation, the names "GP_Layer", "Color", "Lines", and
  "Fills" were not translated at all.
  This adds DATA_() macros around to translate them if the user has
  enabled translation of new data names.
- The functions returning unique names for layers and groups
  unique_layer_group_name() and unique_layer_name() would return
  "GP_Layer" and "GP_Group" by default.
  These defaults are now translated.
- When adding new layers and groups, the name was always hardcoded to
  "GP_Layer" and "GP_Group" because the operators' properties for the
  names defaulted to that, and prop defaults cannot be translated.
  Instead, make them default to a null string, but choose an
  appropriate name when executing the op. This name is chosen by
  unique_layer_name() and unique_layer_group_name(), whose defaults
  are now translated.

Pull Request: https://projects.blender.org/blender/blender/pulls/112930

source/blender/blenkernel/intern/grease_pencil.cc

@@ -1839,13 +1839,13 @@ static std::string unique_node_name(const GreasePencil &grease_pencil,
 static std::string unique_layer_name(const GreasePencil &grease_pencil,
                                      blender::StringRefNull name)
 {
-  return unique_node_name(grease_pencil, "GP_Layer", name);
+  return unique_node_name(grease_pencil, DATA_("GP_Layer"), name);
 }
 
 static std::string unique_layer_group_name(const GreasePencil &grease_pencil,
                                            blender::StringRefNull name)
 {
-  return unique_node_name(grease_pencil, "GP_Group", name);
+  return unique_node_name(grease_pencil, DATA_("GP_Group"), name);
 }
 
 blender::bke::greasepencil::Layer &GreasePencil::add_layer(

source/blender/editors/grease_pencil/intern/grease_pencil_add.cc

@@ -1198,7 +1198,7 @@ void create_blank(Main &bmain, Object &object, const int frame_number)
   int material_index = add_material_from_template(bmain, object, gp_stroke_material_black);
   object.actcol = material_index + 1;
 
-  Layer &new_layer = grease_pencil.add_layer(grease_pencil.root_group(), "GP_Layer");
+  Layer &new_layer = grease_pencil.add_layer(grease_pencil.root_group(), DATA_("GP_Layer"));
   grease_pencil.set_active_layer(&new_layer);
   grease_pencil.insert_blank_frame(new_layer, frame_number, 0, BEZT_KEYTYPE_KEYFRAME);
 }
@@ -1217,8 +1217,8 @@ void create_stroke(Main &bmain, Object &object, float4x4 matrix, const int frame
   add_material_from_template(bmain, object, gp_fill_material_grey);
   object.actcol = material_index + 1;
 
-  Layer &layer_lines = grease_pencil.add_layer(grease_pencil.root_group(), "Lines");
-  Layer &layer_color = grease_pencil.add_layer(grease_pencil.root_group(), "Color");
+  Layer &layer_lines = grease_pencil.add_layer(grease_pencil.root_group(), DATA_("Lines"));
+  Layer &layer_color = grease_pencil.add_layer(grease_pencil.root_group(), DATA_("Color"));
   grease_pencil.set_active_layer(&layer_lines);
 
   grease_pencil.insert_blank_frame(layer_lines, frame_number, 0, BEZT_KEYTYPE_KEYFRAME);
@@ -1278,8 +1278,8 @@ void create_suzanne(Main &bmain, Object &object, float4x4 matrix, const int fram
       color_skin_shadow,
   });
 
-  Layer &layer_fills = grease_pencil.add_layer(grease_pencil.root_group(), "Fills");
-  Layer &layer_lines = grease_pencil.add_layer(grease_pencil.root_group(), "Lines");
+  Layer &layer_fills = grease_pencil.add_layer(grease_pencil.root_group(), DATA_("Fills"));
+  Layer &layer_lines = grease_pencil.add_layer(grease_pencil.root_group(), DATA_("Lines"));
   grease_pencil.set_active_layer(&layer_lines);
 
   grease_pencil.insert_blank_frame(layer_lines, frame_number, 0, BEZT_KEYTYPE_KEYFRAME);

source/blender/editors/grease_pencil/intern/grease_pencil_layers.cc

@@ -83,7 +83,7 @@ static void GREASE_PENCIL_OT_layer_add(wmOperatorType *ot)
   ot->flag = OPTYPE_REGISTER | OPTYPE_UNDO;
 
   PropertyRNA *prop = RNA_def_string(
-      ot->srna, "new_layer_name", "GP_Layer", INT16_MAX, "Name", "Name of the new layer");
+      ot->srna, "new_layer_name", nullptr, INT16_MAX, "Name", "Name of the new layer");
   RNA_def_property_flag(prop, PROP_SKIP_SAVE);
   ot->prop = prop;
 }
@@ -239,12 +239,8 @@ static void GREASE_PENCIL_OT_layer_group_add(wmOperatorType *ot)
 
   ot->flag = OPTYPE_REGISTER | OPTYPE_UNDO;
 
-  PropertyRNA *prop = RNA_def_string(ot->srna,
-                                     "new_layer_group_name",
-                                     "GP_Group",
-                                     INT16_MAX,
-                                     "Name",
-                                     "Name of the new layer group");
+  PropertyRNA *prop = RNA_def_string(
+      ot->srna, "new_layer_group_name", nullptr, INT16_MAX, "Name", "Name of the new layer group");
   RNA_def_property_flag(prop, PROP_SKIP_SAVE);
   ot->prop = prop;
 }

source/blender/editors/interface/interface_template_grease_pencil_layer_tree.cc

@@ -235,7 +235,7 @@ class LayerViewItem : public AbstractTreeViewItem {
   void build_layer_name(uiLayout &row)
   {
     uiBut *but = uiItemL_ex(
-        &row, IFACE_(layer_.name().c_str()), ICON_OUTLINER_DATA_GP_LAYER, false, false);
+        &row, layer_.name().c_str(), ICON_OUTLINER_DATA_GP_LAYER, false, false);
     if (layer_.is_locked() || !layer_.parent_group().is_visible()) {
       UI_but_disable(but, "Layer is locked or not visible");
     }
@@ -337,7 +337,7 @@ class LayerGroupViewItem : public AbstractTreeViewItem {
   void build_layer_group_name(uiLayout &row)
   {
     uiItemS_ex(&row, 0.8f);
-    uiBut *but = uiItemL_ex(&row, IFACE_(group_.name().c_str()), ICON_FILE_FOLDER, false, false);
+    uiBut *but = uiItemL_ex(&row, group_.name().c_str(), ICON_FILE_FOLDER, false, false);
     if (group_.is_locked()) {
       UI_but_disable(but, "Layer Group is locked");
     }