Update the panel plugin system with some of the changes recently made for the desktop plugins.
1) This allows multiple instances of the same plugin on the same toolbar
2) This allows for both horizontal *or* vertical toolbars (left/right/top/bottom)
  Note: Menu's seem to be having issues displaying properly for vertical toolbars. Not sure yet if this is related to something in Lumina or Fluxbox - but they appear to be getting drawn off-screen.
3) While here, fix the sizing issue for the system tray plugin (now it should only use the minimum space it needs).