A couple of improvements to Video VLC implementation (desktop). This com...

This commit is related to issue #34, although it does not fix it apparently (the same "AL lib: (EE) alc_cleanup: 1 device not closed" message still appears). However, this commit makes a couple of improvements to VLC video:

- EngineDesktop has been modified to ensure some good cleaning when the application is closed:
   * It will try to release VLC resources, if created
   * Invokes Gdx.app.exit, instead of calling System.exit directly
Engine.dispose() remains untouched since all these operations seem to be platform-dependent. however, we may want to think a little about setting up a proper dispose protocol (I guess nothing has been really implemented yet)

- I've also wrapped frame.setVisible(true) in EngineDesktop.run() within a SwingUtilities.invokeLater, because in my computer most of the times the Window did not actually show up! This solves that random behaviour.