Fix: UI: Quick tooltip label overridden by custom tooltip function

Using `UI_but_func_tooltip_custom_set()` to create a custom tooltip
would completely disable the quicker label tooltip, even when explicitly
set using `UI_but_func_tooltip_label_set()`. Instead the quick label
should show first, and then the full tooltip once the usual tooltip
timer is reached.

source/blender/editors/interface/regions/interface_region_tooltip.cc

@@ -1523,7 +1523,7 @@ ARegion *UI_tooltip_create_from_button_or_extra_icon(
   }
   std::unique_ptr<uiTooltipData> data = nullptr;
 
-  if (but->tip_custom_func) {
+  if (!is_label && but->tip_custom_func) {
     data = ui_tooltip_data_from_custom_func(C, but);
   }