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
The enums are swapped currently, because for edge scroll is enabled two finger
scroll and similary for two finger scroll is enabled edge scroll, what is
apparently wrong.
https://bugzilla.gnome.org/show_bug.cgi?id=746870