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

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

@@ -63,9 +63,9 @@ struct _MetaInputSettingsClass
   void (* set_invert_scroll) (MetaInputSettings  *settings,
                               ClutterInputDevice *device,
                               gboolean            inverted);
-  void (* set_scroll_method) (MetaInputSettings            *settings,
-                              ClutterInputDevice           *device,
-                              GDesktopTouchpadScrollMethod  mode);
+  void (* set_edge_scroll)   (MetaInputSettings  *settings,
+                              ClutterInputDevice *device,
+                              gboolean            enabled);
   void (* set_scroll_button) (MetaInputSettings  *settings,
                               ClutterInputDevice *device,
                               guint               button);