common: Refactor output-from-settings input device configuration.

The code that handled this when creating a GsdInputInfo and whenever
the settings changed were almost the same, put it in a common place.

https://bugzilla.gnome.org/show_bug.cgi?id=739891
common: Unset output-from-settings if no matching output is found

This ensures that either an output is guessed for the device again, or for
display-less tablets, the default matrix spanning full desktop extents is
reinstaurated.

https://bugzilla.gnome.org/show_bug.cgi?id=739891
common: Update output-from-settings when the outputs change

Outputs being connected/disconnected should update the input devices that
are mapped to these through config, so they are remapped/unmapped correctly
after the change.

https://bugzilla.gnome.org/show_bug.cgi?id=739891