wayland: Check transient loop

wayland: Check transient loop

Gdk Wayland backend walks up the transient windows tree, but does not
check for cycles when doing so.

As a result, if two or more windows are transient to each other, the
Wayland gdk backend will enter an infinite loop.

While this is clearly a bug in the application, gtk+/gdk should be more
robust and handle such errors more gracefully.

To avoid looping infinitely at various point in the code, check for a
possible loop when setting the transient relationship and deny the
request to set a window transient for another if that would create a
loop.

Bugzilla: https://bugzilla.gnome.org/show_bug.cgi?id=759299