stacking: Limit to direct transients when raising

Bug: 15580

Following commit fa9517ea (“Raise all transients together”), xfwm4
would search for the deepest parent of a given transient and raise it to
bring all related windows together.

however, taking into account parents from the same group, it would raise
the lowest window while the others from the same group aren't raised
along with it, causing apparently random windows to be raised.

Make sure we pick only direct transients when walking down the windows
stack.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>