Added dependency TinyXML2 for config file
Added Config class
Modified GameApp constructor
	- Window size is now read from config file
	- Window title is also read from the config file
Added class MainMenuState, it will display the MainMenu to chose from singleplayer and multiplayer and maybe options
Changed statemanager to implement the sf::Drawable interface, and to have an update method
Changed GameApp to use said methods
Implemented fixed time step updating inside GameApp
Added Exception handling around GameApp::run(), MessageBox for Windows, console output for other operating systems
Improved the error handling in StateManager, it throws now runtime errors when the stack is empty