client: Allow transients for group to be minimized

We would not allow transients for group to be minimized as long as a
parent is still visible.

Be more lenient with those, allow those to be minimized independently
of their parents.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>
Closes: #349
2 jobs for master in 3 minutes and 3 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #6737
build

00:01:22

 
  Distcheck
passed #6738
distcheck

00:01:41