[merge request] Bugfix, minor refactoring, etc.

[Teika Kazura <teika lavabit com>]

Hi. Please merge my branch. It contains:
* Minor fix. `this-command' is set to nil when command execution is over.
  Related doc improvement is done, too.
* Minor fix: sawfish-client says "Bye." on normal exit.
* Bugfix. Commands 'maximize-window-fullscreen' and 
  'maximize-window-fullxinerama' work. Related doc error is corrected, too.
* Moved contents of wm/extras.jl to wm/frames.jl, and deleted extras.jl.

Regards,
Teika (Teika kazura)