backends: Force 2-finger scroll by default if available

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