XPrompt: code cleanup
XPrompt: code cleanup

The completion list is not cached anymore: this greatly simplify the code
making its runtime behaviour more predictable...;-) Suggested by Spencer.

darcs-hash:20070803181905-32816-a9a41a08657fff91a279a1bcf362cf72cc0cd458

jthornber/XMonadContrib,jthornber/XMonadContrib