Modifiers: Support applying modifiers for multi-user data

The current behaviour is to prevent multi-user data from having its
modifier applied.

Instead, with this patch, we now warn the user that if they want to
proceed the object will be made single-user.

Note that this only makes the object data single-user. Not the material
or actions.

As a future step we can apply the same behaviour for the Grease Pencil modifiers

Differential Revision: https://developer.blender.org/D14381

source/blender/editors/include/ED_object.h

@@ -381,6 +381,8 @@ struct Object *ED_object_add_type(struct bContext *C,
  */
 void ED_object_single_user(struct Main *bmain, struct Scene *scene, struct Object *ob);
 
+void ED_object_single_obdata_user(struct Main *bmain, struct Scene *scene, struct Object *ob);
+
 /* object motion paths */
 
 /**