system tray: rework a bit, to address #2566 a bit.

- Fixed the sizing issues at the gtk plug level
- simplified the applet
- improved a race condition where the tray icon may have been destroyed
  before the applet had a chance to add it to the panel
     TODO:  this is *still* a race condition - there should be a callback
            that triggers the continuation of the low level na_tray_manager
            handle_dock_Request.