Clean up input files in nifty-renderer-libgdx.

Update comments and remove mapping of Input.Keys.SYM to
KeyboardInputEvent.KEY_SYSRQ because it's incorrect. The SYM key seems
to be the Apple key on OS X, but it is buggy on the libGDX side, so just
map it to nothing for now.
Merge pull request #154 from 3xp0n3nt/libgdx-input-minor-cleanup

Clean up input files in nifty-renderer-libgdx.