Added ability to cycle through average frametime, FPS and average FPS

"t" and "Shift+t" are Lua bindings in common.lua
Maybe needs to be part of SettingsEngine instead, but the display code was already in RenderEngine, so it'll do for now on this branch

include/openspace/engine/wrapper/windowwrapper.h

@@ -79,6 +79,12 @@ public:
      */
     virtual double averageDeltaTime() const;
 
+	/**
+	* Returns the frametime in seconds. On default, this method returns <code>0.0</code>.
+	* \return The frametime in seconds
+	*/
+	virtual double deltaTime() const;
+
     /**
      * Returns the location of the mouse cursor in pixel screen coordinates. On default,
      * this method returns <code>0,0</code>.