Each subset is identified by its "affinity name".
See DockWidget/MainWindow setAffinityName property.
Patch contributed by Andras Mantia, I've changed it to deal in
terms of affinity names instead of unique names.
After a restore with relative sizes the items might not be on a pixel
perfect position regarding to their anchors. Make sure they are after
the restore is complete.
Silenced a false-positive warning about anchor visibility.
It was invisible because we're within a main window show()
It will be checked by the unit-tests and fuzzer anyway. No need to
do it in production.
