IO: Use FileHandlers in the View 3d

Use File Handlers to handle file drag-n-drop in the View 3d. Drop-boxes
still remain since they handle Texture ID drag-n-drop.

This will add-ons to handle drag-n-drop for images and movies while
still providing access to Blender's native support since File Handlers
let users choose which to invoke if there's multiple configured.

Pull Request: https://projects.blender.org/blender/blender/pulls/117728

scripts/startup/bl_operators/view3d.py

@@ -3,7 +3,10 @@
 # SPDX-License-Identifier: GPL-2.0-or-later
 
 import bpy
-from bpy.types import Operator
+from bpy.types import (
+    Operator,
+    FileHandler,
+)
 from bpy.props import (
     BoolProperty,
     EnumProperty,
@@ -258,10 +261,40 @@ class VIEW3D_OT_transform_gizmo_set(Operator):
         return self.execute(context)
 
 
+class VIEW3D_FH_empty_image(FileHandler):
+    bl_idname = "VIEW3D_FH_empty_image"
+    bl_label = "Add empty image"
+    bl_import_operator = "OBJECT_OT_empty_image_add"
+    bl_file_extensions = ';'.join(bpy.path.extensions_image) + ';' + ';'.join(bpy.path.extensions_movie)
+
+    @classmethod
+    def poll_drop(cls, context):
+        if not context.space_data or context.space_data.type != 'VIEW_3D':
+            return False
+        rv3d = context.space_data.region_3d
+        return rv3d.view_perspective == 'PERSP' or rv3d.view_perspective == 'ORTHO'
+
+
+class VIEW3D_FH_camera_background_image(FileHandler):
+    bl_idname = "VIEW3D_FH_camera_background_image"
+    bl_label = "Add camera background image"
+    bl_import_operator = "VIEW3D_OT_camera_background_image_add"
+    bl_file_extensions = ';'.join(bpy.path.extensions_image) + ';' + ';'.join(bpy.path.extensions_movie)
+
+    @classmethod
+    def poll_drop(cls, context):
+        if not context.space_data or context.space_data.type != 'VIEW_3D':
+            return False
+        rv3d = context.space_data.region_3d
+        return rv3d.view_perspective == 'CAMERA'
+
+
 classes = (
     VIEW3D_OT_edit_mesh_extrude_individual_move,
     VIEW3D_OT_edit_mesh_extrude_move,
     VIEW3D_OT_edit_mesh_extrude_shrink_fatten,
     VIEW3D_OT_edit_mesh_extrude_manifold_normal,
     VIEW3D_OT_transform_gizmo_set,
+    VIEW3D_FH_camera_background_image,
+    VIEW3D_FH_empty_image,
 )

source/blender/editors/space_view3d/space_view3d.cc

@@ -620,11 +620,6 @@ static bool view3d_ima_drop_poll(bContext *C, wmDrag *drag, const wmEvent *event
   if (ED_region_overlap_isect_any_xy(CTX_wm_area(C), event->xy)) {
     return false;
   }
-  if (drag->type == WM_DRAG_PATH) {
-    const eFileSel_File_Types file_type = eFileSel_File_Types(WM_drag_get_path_file_type(drag));
-    return ELEM(file_type, FILE_TYPE_IMAGE, FILE_TYPE_MOVIE);
-  }
-
   return WM_drag_is_ID_type(drag, ID_IM);
 }
 
@@ -887,11 +882,6 @@ static void view3d_id_path_drop_copy(bContext *C, wmDrag *drag, wmDropBox *drop)
     RNA_struct_property_unset(drop->ptr, "filepath");
     return;
   }
-  const char *path = WM_drag_get_single_path(drag);
-  if (path) {
-    RNA_string_set(drop->ptr, "filepath", path);
-    RNA_struct_property_unset(drop->ptr, "image");
-  }
 }
 
 static void view3d_lightcache_update(bContext *C)