Arbitrary instance for StackSet must set random focus on each workspace

When focus was separated from the stack order on each workspace, we
forgot to update the Arbitrary instance to set random focus. As spotted
by David R, this then invalidates 4 of our QC properties. In particular,
the property involving where focus goes after a random transient
(annoying behaviour) appeared to be correct, but wasn't, due to
inadequate coverage.

This patch sets focus to a random window on each workspace. As a result,
we now catch the focus/raise/delete issue people have been complaining
about.

Lesson: make sure your QuickCheck generators are doing what you think
they are.

darcs-hash:20070508051126-9c5c1-55a0597e6838b35f12b3d348e85360cda6c237ca

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