Remove unused GestureTracker (#2304)

Author: leolost2605

src/Gestures/GestureBackend.vala

@@ -0,0 +1,23 @@
+/*
+ * Copyright 2025 elementary, Inc. (https://elementary.io)
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Authored by: Leonhard Kargl <[email protected]>
+ */
+
+public interface Gala.GestureBackend : Object {
+    public signal bool on_gesture_detected (Gesture gesture, uint32 timestamp);
+    public signal void on_begin (double delta, uint64 time);
+    public signal void on_update (double delta, uint64 time);
+    public signal void on_end (double delta, uint64 time);
+
+    public virtual void prepare_gesture_handling () { }
+
+    /**
+     * The gesture should be cancelled. The implementation should stop emitting
+     * signals and reset any internal state. In particular it should not emit on_end.
+     * The implementation has to make sure that any further events from the same gesture will
+     * will be ignored. Once the gesture ends a new gesture should be treated as usual.
+     */
+    public virtual void cancel_gesture () { }
+}