window: Raise and lower tile match in tandem

When a pair of tiled windows are grouped together, they
are treated as parts of a whole and interacting with one
affects the other.

Following the idea that sibling tiled windows are treated
as part of the same group, they should also be raised and
lowered together.

It is still possible to break tiled windows grouping by
simply untiling the window with the keyboard or by grabbing
and resizing or moving the window with the cursor.

This patch makes sibling tiled windows be lowered and raised
in tandem. For future reference, this behavior is documented
in [1].

[1] https://wiki.gnome.org/GeorgesNeto/MinutesOfFeaneron/Tiling

https://bugzilla.gnome.org/show_bug.cgi?id=645153