common: Fix possible crash in GsdDeviceMapper

If the input device isn't known to the device mapper (say a keyboard for
example), we'd crash trying to check which output it was attached to.

https://bugzilla.gnome.org/show_bug.cgi?id=720295
power: Disable touchscreens when turning off its screen

For example, turning off a tablet's touchscreen when the screenlock
kicks in.

Note that this is currently broken due to bugs in the kernel.
For the X.org tracking bug, see
https://bugs.freedesktop.org/show_bug.cgi?id=81781

And a possible patch exists at:
https://lkml.org/lkml/2014/9/30/588

https://bugzilla.gnome.org/show_bug.cgi?id=720295