backends: Have meta_input_settings_handle_pad_button() take an event

As all the relevant backends are expected to provide
ClutterPadButtonEvents, it makes no sense to split the information,
plus all other event fields are now available and might be needed
in the future.

https://bugzilla.gnome.org/show_bug.cgi?id=771098

src/core/events.c

@@ -223,9 +223,7 @@ meta_display_handle_event (MetaDisplay        *display,
        event->type == CLUTTER_PAD_BUTTON_RELEASE))
     {
       if (meta_input_settings_handle_pad_button (meta_backend_get_input_settings (backend),
-                                                 clutter_event_get_source_device (event),
-                                                 event->type == CLUTTER_PAD_BUTTON_PRESS,
-                                                 event->pad_button.button))
+                                                 &event->pad_button))
         {
           bypass_wayland = bypass_clutter = TRUE;
           goto out;