Major hacking on the DawkinsSystemTray.

- Added dawkins_system_tray_{manage,unmanage} functions to allow
  the application to control when and which screen to manage.
- Added screen (readonly), visual, and orientation properties
  to the DawkinsSystemTray. The visual and orientation are set
  as window hints, as per
  http://standards.freedesktop.org/systemtray-spec/systemtray-spec-0.3.html.