wayland: Maybe postpone xdg-foreign state setup until mapping

wayland: Maybe postpone xdg-foreign state setup until mapping

In order to map a window with the correct initial parent-child
relationship when a modal dialog is set up to be a child of an imported
foreign window, the relationship must be set up before the window is
mapped.

In order to do this, if a window is not yet mapped, postpone the
relationship setup until when the window is eventually mapped.

https://bugzilla.gnome.org/show_bug.cgi?id=791062