placement: Use a GdkRectangle instead of multiple coords

Pass a reference to a GdkRectangle instead of multiple references to
x/y/w/h.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>
2 jobs for !97 with move-to-monitor-various-fixes in 2 minutes and 35 seconds (queued for 8 seconds)
merge request