XPrompt: fixes a couple of bugs
XPrompt: fixes a couple of bugs

- we run the action passed to mkXPrompt only if we have a command;
- updateWindows must call destroyComplWin if there are no completions;
- some comments (more to come)
- a shorthand in keyPressHandle
- removed all warnings

darcs-hash:20070804090817-32816-6440f7fce4d69e6738855c1159384d822a6ab51f

yzhou61/xmonad-contrib,yzhou61/xmonad-contrib,yzhou61/xmonad-contrib