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

src/backends/meta-input-settings-private.h

@@ -71,6 +71,9 @@ struct _MetaInputSettingsClass
   void (* set_edge_scroll)   (MetaInputSettings  *settings,
                               ClutterInputDevice *device,
                               gboolean            enabled);
+  void (* set_two_finger_scroll) (MetaInputSettings  *settings,
+                                  ClutterInputDevice *device,
+                                  gboolean            enabled);
   void (* set_scroll_button) (MetaInputSettings  *settings,
                               ClutterInputDevice *device,
                               guint               button);