clutter/event: Add ClutterEventType.CLUTTER_TOUCHPAD_HOLD

Add a enum for hold gestures in ClutterEventType as well as the
required functions to get information about the event: coordinates,
finger count, event phase, etc.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1830>

src/core/events.c

@@ -43,6 +43,7 @@
 
 #define IS_GESTURE_EVENT(e) ((e)->type == CLUTTER_TOUCHPAD_SWIPE || \
                              (e)->type == CLUTTER_TOUCHPAD_PINCH || \
+                             (e)->type == CLUTTER_TOUCHPAD_HOLD || \
                              (e)->type == CLUTTER_TOUCH_BEGIN || \
                              (e)->type == CLUTTER_TOUCH_UPDATE || \
                              (e)->type == CLUTTER_TOUCH_END || \