T90371: Asset: Drop Material Tooltip.

This patch changes the drop named material tooltip to give feedback to
the user what is going to happen when they invoke the change.

There are 3 states:
* "": Operator will be canceled as not all data is present (dropping on
  background.)
* "Drop <named material> on <object name> (slot <slot number>, replacing
  <current material in slot>).
* "Drop <named material> on <object name> (slot <slot number).

Reviewed By: Severin

Maniphest Tasks: T90371

Differential Revision: https://developer.blender.org/D12106

source/blender/editors/include/ED_object.h

@@ -53,6 +53,7 @@ struct uiLayout;
 struct wmKeyConfig;
 struct wmOperator;
 struct wmOperatorType;
+struct wmEvent;
 
 /* object_edit.c */
 /* context.object */
@@ -199,6 +200,9 @@ void ED_object_parent(struct Object *ob,
                       struct Object *parent,
                       const int type,
                       const char *substr);
+char *ED_object_ot_drop_named_material_tooltip(struct bContext *C,
+                                               struct PointerRNA *properties,
+                                               const struct wmEvent *event);
 
 /* bitflags for enter/exit editmode */
 enum {