Add missing file in 67eeac8045

src/app/ui/timeline/timeline_observer.h

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+// Aseprite
+// Copyright (C) 2021  Igara Studio S.A.
+//
+// This program is distributed under the terms of
+// the End-User License Agreement for Aseprite.
+
+#ifndef APP_UI_TIMELINE_TIMELINE_OBSERVER_H_INCLUDED
+#define APP_UI_TIMELINE_TIMELINE_OBSERVER_H_INCLUDED
+#pragma once
+
+namespace app {
+  class Timeline;
+
+  class TimelineObserver {
+  public:
+    virtual ~TimelineObserver() { }
+
+    // Called when the current timeline range is going to change.
+    virtual void onBeforeRangeChanged(Timeline* timeline) { }
+  };
+
+} // namespace app
+
+#endif