Rendering ImGui, ScreenLog, and Information as a post-processing step to make it work in fisheye rendering (closing #119)

Disable SGCT ESC terminate handling and implement a method that optionally waits before closing the application (closing #120)
 - ESC key is now bound to a call to "openspace.toggleShutdown()" that will initiate a shutdown in 3 seconds unless ESC is pressed again
 - The countdown timer is set in the openspace.cfg

include/openspace/engine/wrapper/sgctwindowwrapper.h

@@ -36,6 +36,7 @@ namespace openspace {
  */
 class SGCTWindowWrapper : public WindowWrapper {
 public:
+    void terminate() override;
     void setBarrier(bool enabled) override;
     void clearAllWindows(const glm::vec4& clearColor) override;
     bool windowHasResized() const override;

include/openspace/engine/wrapper/windowwrapper.h

@@ -42,6 +42,12 @@ namespace openspace {
  */
 class WindowWrapper {
 public:
+    /**
+     * This method closes the application by calling the necessary terminate function of
+     * the window management system
+     */
+    virtual void terminate();
+
     /**
      * This method enables or disables a framelock barrier. If the specific windowing
      * framework does not provide a framelock, this method defaults to a no-op.