x11-display: Handle mapped XIDs per type

Starting from commit 7713006f, during X11 disposition we also unmanage the
windows using the xids hash table values list.
However, this is also populated by the X11 Meta barrier implementation and then
contains both Windows and Barriers.

So when going through the values list, check whether we're handling a window or
a barrier and based on that, unmanage or destroy it.

Fixes GNOME/mutter#624
GNOME/mutter!605
4 jobs for !605 with display-disposition-fixes in 5 minutes and 58 seconds (queued for 1 second)
latest detached
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  Review
passed #319975
check-commit-log

00:00:26

 
  Build
passed #319976
build-mutter

00:02:39

 
  Test
passed #319978
can-build-gnome-shell

00:01:37

passed #319977
test-mutter

00:02:51