issue #188

Addressing issue #188 at https://code.google.com/p/libgdx/issues/detail?id=188

Make `LwjglApplication` paused when minimized or not on focus, and call resume when back on focus. It configurable with `LwjglApplicationConfiguration.pauseWhenMinimized`. By default the value is `false`, which means it will not pause when the application minimized, and the application work like this code never added. When it set to `true`, the loop will call `listener.pause()` and skip every rendering parts when the application minimized, and call `listener.resume()` when it's back on focus.