IsWindowBeingClosed reports whether the window is being closed by
the user.
SetWindowClosingHandled sets whether the window closing is handled
or not. If the state is true, the window is not closed immediately
by the user and the game can handle the closing state. In this case,
the Update function should return an error in order to end the game.
This change also adds examples/windowclosing.
Closes#1574
