Track clients connected to each device using EggDbusMonitor.
When there are no more clients connected to the device, release it.
When no devices are used anymore, set up a timeout to exit within
30 seconds.
Get a PolicyKit context per-device, set up its main loop,
and steal more code from gnome-panel to check whether
the actions are allowed for a particular caller.
This will be needed to get better error messages to
users of the D-Bus API. We use glib's builtin gettext
support. No strings marked as translatable yet, we'll
need to mark those user-visible strings carefully and
review them.
