backends: Add API to trigger actions related to pad buttons

It does nothing at the moment, but can be hooked into MetaWaylandTabletPad
now. For X11, we need to trigger these for the pad events we receive from
the passive pad button grabs.

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

@@ -115,6 +115,15 @@ gdouble                    meta_input_settings_translate_tablet_tool_pressure (M
                                                                                ClutterInputDevice     *current_tablet,
                                                                                gdouble                 pressure);
 
+gboolean                   meta_input_settings_is_pad_button_grabbed     (MetaInputSettings  *input_settings,
+                                                                          ClutterInputDevice *pad,
+                                                                          guint               button);
+
+gboolean                   meta_input_settings_handle_pad_button         (MetaInputSettings  *input_settings,
+                                                                          ClutterInputDevice *pad,
+                                                                          gboolean            is_press,
+                                                                          guint               button);
+
 #ifdef HAVE_LIBWACOM
 WacomDevice * meta_input_settings_get_tablet_wacom_device (MetaInputSettings *settings,
                                                            ClutterInputDevice *device);