Marco Trevisan authored Jun 05, 2019 and Marco Trevisan committed Jun 06, 2019

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 #624
!605