Get the parent window XID for the email portal

Here I wrote that we were unable to get the parent window XID from the receive class. https://github.com/gnome-keysign/gnome-keysign/blob/34e898d84af89cb5f7e74e357aaab139493a4989/keysign/util.py#L78

Actually this was due to the fact that I tried to get the parent before the complete initialization of the class. So with get_toplevel() we should be able to get the parent window that we need in util.

This should be only a matter of getting this element and passing it from receive to util.

Also interesting, I guess, is this thread: https://lists.freedesktop.org/archives/flatpak/2019-July/001633.html

Wayland surfaces don't have a public ID unless you explicitly create one using the zxdg_exporter protocol. The client-side api for this is gdk_wayland_window_export_handle. If this seems too much hassle, just passing an empty string will work fine for most purposes.