When the touchpad is two-finger scrolling capable, always enable it.
When the touchpad only supports edge scrolling (usually older devices, and
usually smaller devices), allow disabling the edge scrolling.
This requires a newer gsettings-desktop-schemas as the scroll-method key
was removed, and the edge-scroll-enabled key added.
https://bugzilla.gnome.org/show_bug.cgi?id=759304
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
