xgame: An update

I've done very little poking around on gravitas in the last year, my SVN repo only saw commits in September last year and March. Recent changes:

• Physics works well enough to set up orbits.
• New models. (for the observant, the red planet is the ball from the dx samples)

XNA 2.0 might be part of the reason for stalling, I've been pondering the move to there.