Window: Close window when unsetting transient_for
Since b443bd42, we unmanage a wayland window when clearing its transient parent. That's to make sure that xdg-foreign doesn't leave the dialog around after the imported surface was destroyed.
While that behavior is sound, it is problematic to implement it by unmanaging the window, as that happens entirely behind the client's back.
Instead, send a close event for the window. Unless the client has good reasons, it should honor the request. (And if it has good reasons - like unsaved work - then effectively hiding the window from both the user and client is probably not the best idea anyway).