seat: Listen for ClutterDeviceManager signals in order to update capabilities

The capability flags are determined from the device types of the slave devices
that are currently attached. This also happens whenever a device is added or
removed, so the capabilities are kept up to date, and clients know about these.

On VT switch, all slave devices are temporarily removed, so the cascade of
signals will make the seat end up with capabililities=0 while input is suspended.

https://bugzilla.gnome.org/show_bug.cgi?id=733563