On Wayland, xdg-foreign would leave a modal dialog managed even after
the imported surface is destroyed.
This is sub-optimal and this breaks the atomic relationship one would
expect between the parent and its modal dialog.
Make sure we unmanage the dialog if transient_for is unset even for
Wayland native windows.
Related: https://gitlab.gnome.org/GNOME/mutter/issues/174
Related: https://gitlab.gnome.org/GNOME/mutter/issues/221
(cherry picked from commit b443bd42ac738092817addf48e0b363d140cad26)
398f03bd0f