System tray integration take 5

[Mike Hearn]

MikeM gave me some debugging tips, which resulted in a fix for the nasty
heisenbug. At least, I can't reproduce it anymore, no matter how many
times I click the button.

This patch also rolls up the patch that put the system tray in its own
thread.

Obviously, it obsoletes the last one I sent.

ChangeLog:
Support the XEMBED system tray protocol
Run system tray in a separate thread

thanks (for your patience ;) -mike