xfdesktop needs to handle thunar uca differently now

Issue Description

With the recent Thunar commit that made thunar-uca an internal function rather than a thunarx plugin is causing xfdesktop to not function for thunar uca actions. The custom actions are missing because xfdesktop is looking for the thunarx plugin which no longer exists. Thunar version 4.21.6-dev xfdesktop version 4.21.0-dev

Steps to Reproduce Issue

upgrade thunar to the latest git version

Expected Behavior

Thunar uca shows up in the xfdesktop context menu as expected