MetaInputSettings: allow edge scrolling without 2fg capable devices

We should only force edge scrolling off if two finger is enabled *and*
we actually have two finger capable devices.

https://bugzilla.gnome.org/show_bug.cgi?id=778554

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

@@ -107,6 +107,9 @@ struct _MetaInputSettingsClass
   void (* set_trackball_accel_profile) (MetaInputSettings          *settings,
                                         ClutterInputDevice         *device,
                                         GDesktopPointerAccelProfile profile);
+
+  gboolean (* has_two_finger_scroll) (MetaInputSettings  *settings,
+                                      ClutterInputDevice *device);
 };
 
 GType meta_input_settings_get_type (void) G_GNUC_CONST;