Brightness in frame try2

**This is a feature and should not be merged until after the freeze**

This adds a brightness model and screenshot model (from existing screenshot code).  This is then used to create a display frame icon, which houses the brightness and screenshot.

The brightness thing is a bit weird and has limitations.  It either manipulates the file directly or if uses GNOME SettingsDaemon over dbus.  If none of theses are writable, you get a read only brightness in the frame.