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

src/engine/wrapper/windowwrapper.cpp

@@ -31,6 +31,8 @@ namespace openspace {
 WindowWrapper::WindowWrapperException::WindowWrapperException(const std::string& msg)
     : ghoul::RuntimeError(msg, "WindowWrapper")
 {}
+
+void WindowWrapper::terminate() {}
     
 void WindowWrapper::setBarrier(bool) {}