I18n: Disambiguate "Extend"

- "Extend" usually means making selection bigger around the current
one.
- In the context of Grease Pencil filling, it means lengthen lines.
- In the context of curve mappings, it refers to the extrapolation
  mode.
- In the context of volume sequences, it refers to frame range
extension.

Issue reported by Hoang Duy Tran.

source/blender/editors/sculpt_paint/grease_pencil_draw_ops.cc

@@ -1059,7 +1059,7 @@ static void grease_pencil_fill_status_indicators(bContext &C,
       fmt::runtime(
           IFACE_("Fill: ESC/RMB cancel, LMB Fill, MMB Adjust Extension, S: "
                  "Switch Mode, D: Stroke Collision | Mode: {}, Collision {}, Length: {:.3f}")),
-      (is_extend) ? IFACE_("Extend") : IFACE_("Radius"),
+      (is_extend) ? CTX_IFACE_(BLT_I18NCONTEXT_ID_GPENCIL, "Extend") : IFACE_("Radius"),
       (is_extend && op_data.extension_cut) ? IFACE_("ON") : IFACE_("OFF"),
       op_data.extension_length);
 

source/blender/makesrna/intern/rna_brush.cc

@@ -1725,6 +1725,7 @@ static void rna_def_gpencil_options(BlenderRNA *brna)
   RNA_def_property_enum_items(prop, rna_enum_gpencil_fill_extend_modes_items);
   RNA_def_property_ui_text(
       prop, "Closure Mode", "Types of stroke extensions used for closing gaps");
+  RNA_def_property_translation_context(prop, BLT_I18NCONTEXT_ID_GPENCIL);
   RNA_def_parameter_clear_flags(prop, PROP_ANIMATABLE, ParameterFlag(0));
   RNA_def_property_update(prop, 0, "rna_BrushGpencilSettings_update");
 

source/blender/makesrna/intern/rna_color.cc

@@ -942,6 +942,7 @@ static void rna_def_curvemapping(BlenderRNA *brna)
   RNA_def_property_enum_bitflag_sdna(prop, nullptr, "flag");
   RNA_def_property_enum_items(prop, prop_extend_items);
   RNA_def_property_ui_text(prop, "Extend", "Extrapolate the curve or extend it horizontally");
+  RNA_def_property_translation_context(prop, BLT_I18NCONTEXT_ID_CURVE_LEGACY);
   RNA_def_property_update(prop, 0, "rna_CurveMapping_extend_update");
 
   prop = RNA_def_property(srna, "curves", PROP_COLLECTION, PROP_NONE);

source/blender/makesrna/intern/rna_volume.cc

@@ -624,6 +624,7 @@ static void rna_def_volume(BlenderRNA *brna)
   RNA_def_property_clear_flag(prop, PROP_ANIMATABLE);
   RNA_def_property_enum_items(prop, sequence_mode_items);
   RNA_def_property_ui_text(prop, "Sequence Mode", "Sequence playback mode");
+  RNA_def_property_translation_context(prop, BLT_I18NCONTEXT_ID_SEQUENCE);
   RNA_def_property_update(prop, 0, "rna_Volume_update_filepath");
 
   /* Grids */