UI: support A-Z accelerator keys for pie menus

Converting menus to PIE's was removing convenient key accelerators.

source/blender/editors/interface/interface.c

@@ -979,7 +979,9 @@ static void ui_menu_block_set_keyaccels(uiBlock *block)
                 UI_BTYPE_BUT_MENU,
                 UI_BTYPE_MENU,
                 UI_BTYPE_BLOCK,
-                UI_BTYPE_PULLDOWN) ||
+                UI_BTYPE_PULLDOWN,
+                /* For PIE-menus. */
+                UI_BTYPE_ROW) ||
           (but->flag & UI_HIDDEN)) {
         /* pass */
       }