Add some features to GlfwApp

• Added toggleFullscreen method to enter / leave borderless fullscreen, added isFullscreen getter. Restores the previous window position and size after leaving fullscreen
• GlfwApp now tracks if the main window is minimized (Glfw term: iconified), skips rendering if it is. Added isMinimized getter. This fixes error spam while the window is minimized in windows (incomplete framebuffer warnings)
• Added requestClose method to shut down the application as if the window had been closed
• Added optional secondary, aliased OpenGL context, will be created if setCreateSecondaryContext(true) is called (defaults to false). Can be retrieved using secondaryContext(), analogous to existing window() method. Useful for multithreaded interaction with OpenGL, like asset loading
• Added optional virtual scheduledUpdate method which is called in the interval specified by calling setScheduledUpdateInterval (defaults to 0, which means never). Useful to perform clean up tasks
• Fixed some unused parameter and data type warnings