compositor: Focus the no focus window during modal plugin operations

Modal operations are usually far from transient, so it makes sense to
unfocus the currently focused window while the operation is going on.
If no other window was focused and the window is still alive when the
modal operation is finished, focus will be restored back.

Closes: gnome-shell#452
3 jobs for !233 with wip/carlosg/unfocus-on-modal in 32 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Review
failed #221079
check-commit-log

00:00:32

 
  Build
skipped #221080
build-mutter
 
  Test
skipped #221081
test-mutter
 
Name Stage Failure
failed
check-commit-log Review
Cloning repository...
Cloning into '/builds/GNOME/mutter'...
Checking out e62c9e1c as wip/carlosg/unfocus-on-modal...
Skipping Git submodules setup
$ ./.gitlab-ci/check-commit-log.sh
From https://gitlab.gnome.org/GNOME/mutter
* branch master -> FETCH_HEAD
Missing merge request or issue URL on commit e62c9e1c
ERROR: Job failed: exit code 1