* xeventUnmap() did not check the union type before reading from it.
* unmap event should always come, no need to sleep/retry 30 times.
* remove xeventUnmap() and waitUnmapWindowNotify() entirely. the main
  logic is short enough to inline directly at the call site.
* rather than unmapping the window, and then destroying it, destroy it
  directly and wait for the DestroyNotify event.
* compare window ID against `None` rather than `0`. no functional
  difference, but using `None` is more idiomatic for X11 things.

this commit also fixes a regression introduced in b2d4358. because we
pull the event out now, the DestroyNotify event won't stick around in
the queue anymore.

Fixes: resurrecting-open-source-projects#319

Fixes: resurrecting-open-source-projects#304 (comment)