Revert "Don't unmap or withdraw dying windows"

This reverts commit 6577649bbb457527912e9894752333a4cd934331.

In fact, we *do* want to unmap and withdraw these windows, since they aren't
all destroyed e.g. by things which have a systray icon and activate their
"main" window by clicking on the systray icon. These windows are never
destroyed, but should be unmaped and withdrawn.

Fixes #477