Minor LibGDX examples cleanup.

- Upgrade Nifty version in LibGDX examples. Pom value in nifty-examples-libgdx
  pom incorrectly specifies 1.4.0. Change to 1.4.1-SNAPSHOT.

- Fix LibGDX android examples. Remove nonexistent configuration value
  'useGL20'.
Add music looping to GdxMusicHandle.