Adjust how the backend for the system tray works:
1) Now the Lumina session registeres the system-wide tray, and just keeps track of the windows that should be visible (preventing any loss of service or apps starting up before the tray is registered).
2) The system tray plugin is now just a "visual tray" for embedding the applications in the panel. The Session only allows a single visual tray to be registered at a time, but sends out signals whenever there is an opening for a visual tray (allowing near-instant transfer of tray applications from one visual tray to another).

Also fix a quick bug in lumina-config where the save button was getting activated simply by changing the appearance/plugins tab in the panel configuration (without actually changing anything).