Add ESC key support to exit application in InputService

src/main/java/be/seeseemelk/diceos/system/InputService.java

@@ -23,6 +23,8 @@ public class InputService implements OnStartup {
                 log.info("Key pressed: {}", keycode);
                 if (keycode == Input.Keys.ENTER && isAltDown()) {
                     displayService.toggleFullscreen();
+                } else if (keycode == Input.Keys.ESCAPE) {
+                    Gdx.app.exit();
                 }
                 return false;
             }