backends: Re-add support for edge scrolling with some touchpads

Add support for setting edge-scrolling separately from two-finger
scrolling. We now have 2 separate boolean settings for those, with the
Mouse panel in gnome-control-center allowing to set only one of those at
a time, but nothing precludes both being set in the configuration.

We need to handle:
- two-finger-scrolling-enabled and edge-scrolling-enabled settings both
  being set.
- those 2 settings being change out-of-order
- two-finger-scrolling being set on a device that doesn't support it
- edge-scrolling-enabled on a device that doesn't support it

And the combinations of one touchpad supporting just one of edge
scrolling and two-finger scrolling and another vice-versa.

https://bugzilla.gnome.org/show_bug.cgi?id=768245