This object internally keeps track of the relevant input configuration,
and goes through its vmethods in order to apply the configuration on the
backend-specific devices.
So far, only mouse/touchpad settings are actually attached to GSettings
changes. ::set_matrix(), meant for tablets/touchscreens, is not hooked
yet.
One caveat is that meta_input_settings_create() may return NULL if the
backend does not own the windowing system (wayland nested on X11 being
the one case), and thus device settings can't be changed freely.
https://bugzilla.gnome.org/show_bug.cgi?id=739397
