UI: add menu search functionality to operator search menu

This has some advantages over operator search:

- Some operators need options set to be usefully accessed.
- Shows key bindings to access menus
  (for actions that don't have key bindings themselves).
- Non operator actions such as check-boxes are also shown.
- Menu items can control execution context, using invoke or execute
  where appropriate so we can control how the operator runs.

Part of the design task T74157.

This can be tested using the 'Experimental' preferences section
or selected in the key-map editor.

source/blender/editors/interface/interface_intern.h

@@ -203,8 +203,8 @@ struct uiBut {
 
   uiButSearchCreateFunc search_create_func;
   uiButSearchFunc search_func;
-  bool free_search_arg;
   void *search_arg;
+  uiButSearchArgFreeFunc search_arg_free_func;
 
   uiButHandleRenameFunc rename_func;
   void *rename_arg1;
@@ -649,6 +649,8 @@ ColorPicker *ui_block_colorpicker_create(struct uiBlock *block);
 /* Searchbox for string button */
 ARegion *ui_searchbox_create_generic(struct bContext *C, struct ARegion *butregion, uiBut *but);
 ARegion *ui_searchbox_create_operator(struct bContext *C, struct ARegion *butregion, uiBut *but);
+ARegion *ui_searchbox_create_menu(struct bContext *C, struct ARegion *butregion, uiBut *but);
+
 bool ui_searchbox_inside(struct ARegion *region, int x, int y);
 int ui_searchbox_find_index(struct ARegion *region, const char *name);
 void ui_searchbox_update(struct bContext *C, struct ARegion *region, uiBut *but, const bool reset);