UI: Display disabled-hint when dragging material outside object mode

Display a "disabled hint" (text explaining why something isn't possible)
when dragging a material over the 3D View, while being in edit mode or
so (anything that isn't object mode).

source/blender/editors/object/object_relations.c

@@ -2656,7 +2656,7 @@ void OBJECT_OT_drop_named_material(wmOperatorType *ot)
 
   /* api callbacks */
   ot->invoke = drop_named_material_invoke;
-  ot->poll = ED_operator_objectmode;
+  ot->poll = ED_operator_objectmode_poll_msg;
 
   /* flags */
   ot->flag = OPTYPE_UNDO | OPTYPE_INTERNAL;