Event System (#1741)

* Add implementation of the EventEngine to handle global event chains
* Add properties to SceneGraphNodes to determine two distance radii for camera-based events

include/openspace/navigation/navigationhandler.h

@@ -140,6 +140,7 @@ public:
 
 private:
     void applyNavigationState(const NavigationState& ns);
+    void updateCameraTransitions();
 
     bool _playbackModeEnabled = false;
 
@@ -147,6 +148,10 @@ private:
     Camera* _camera = nullptr;
     std::function<void()> _playbackEndCallback;
 
+    inline static const double InteractionHystersis = 0.0125;
+    bool _inAnchorApproachSphere = false;
+    bool _inAnchorReachSphere = false;
+
     OrbitalNavigator _orbitalNavigator;
     KeyframeNavigator _keyframeNavigator;
     PathNavigator _pathNavigator;