backends: Expose InputMapping D-Bus path to determine tablet mapping

Currently, the peripheral "output" setting will be unset if Mutter is deciding automatically the mapped output of a tablet device. In that case, gnome-control-center will have a hard time figuring out itself the better output to show the tablet calibration UI, unless it's hand held by Mutter.

Add this private D-Bus interface so that gnome-control-center can look up the output as determined by Mutter to bring the missing harmony between both. This interface consists of a simple method to get the mapped output for a input device node.