Fix T53630: Effect strips not displaying Input data.

Fix T52977: Parent bone name disappeared in the UI in pose mode.

Regression caused by own rBc57636f060018. So instead of changing widget
type, just flag it as disabled.

Note that core of the issue is elsewhere though - there is absolutely no
reasons to have a search widget for pointers we cannot change nor
search! But fixing this is not really top priority, one of the many
glitches of our UI code, so think we can live with current code.

To be backported to 2.79a.

source/blender/editors/interface/interface_layout.c

@@ -1787,7 +1787,7 @@ void ui_but_add_search(uiBut *but, PointerRNA *ptr, PropertyRNA *prop, PointerRN
 	}
 	else if (but->type == UI_BTYPE_SEARCH_MENU) {
 		/* In case we fail to find proper searchprop, so other code might have already set but->type to search menu... */
-		but->type = UI_BTYPE_LABEL;
+		but->flag |= UI_BUT_DISABLED;
 	}
 }