Multi-Object Editing

This adds initial multi-object editing support.

- Selected objects are used when entering edit & pose modes.
- Selection & tools work on all objects however many tools need porting
  See: T54641 for remaining tasks.

Indentation will be done separately.

See patch: D3101

source/blender/editors/include/ED_object.h

@@ -124,10 +124,13 @@ enum {
 	EM_WAITCURSOR       = (1 << 1),
 	EM_DO_UNDO          = (1 << 2),
 	EM_IGNORE_LAYER     = (1 << 3),
+	EM_NO_CONTEXT       = (1 << 4),
 };
 void ED_object_editmode_exit_ex(
         struct bContext *C, struct Scene *scene, struct Object *obedit, int flag);
 void ED_object_editmode_exit(struct bContext *C, int flag);
+
+void ED_object_editmode_enter_ex(struct Scene *scene, struct Object *ob, int flag);
 void ED_object_editmode_enter(struct bContext *C, int flag);
 bool ED_object_editmode_load(struct Object *obedit);