x11: add support for _NET_WORKAREAS_Dn

If window manager supports _NET_WORKAREAS use per-monitor work areas.

https://mail.gnome.org/archives/wm-spec-list/2018-December/msg00000.html
2 jobs for wip/muktupavels/work-areas-v3-3-24 in 6 minutes and 16 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Build
passed #196280
build

00:06:16

manual #196281
win32 allowed to fail manual
msys2-mingw32