Fix #147752: Button swap is missing custom tooltip callbacks

Tooltips could crash where callbacks receive the wrong argument.
This is because the swap function was missing the `custom_tip_func` callback,
but was swapping the callback argument.

Pull Request: https://projects.blender.org/blender/blender/pulls/147806

source/blender/editors/interface/interface.cc

@@ -972,6 +972,7 @@ static void ui_but_update_old_active_from_new(uiBut *oldbut, uiBut *but)
   /* Move tooltip from new to old. */
   std::swap(oldbut->tip_func, but->tip_func);
   std::swap(oldbut->tip_arg, but->tip_arg);
+  std::swap(oldbut->tip_custom_func, but->tip_custom_func);
   std::swap(oldbut->tip_arg_free, but->tip_arg_free);
   std::swap(oldbut->tip_quick_func, but->tip_quick_func);