This handle is one of the opaque window handles as returned by
gimp_dialog_get_native_handle() or gimp_progress_get_window_handle() and
therefore it works even across processes. Now any export procedure using
the GimpExportProcedureDialog will call "plug-in-metadata-editor" as a
transient window of itself when clicking the "Metadata (edit)" button.
In particular, the Metadata editor won't be hidden by mistake when
appearing or similar issues.
Nevertheless it only works for systems where we have a transient window
implementation (so far, only X11 and Wayland, since the Windows
implementation is currently commented out because of #10229, and we have
no macOS implementation).
Note though that setting destroy-with-parent doesn't work, most likely
because the GimpExportProcedureDialog is still waiting for the Metadata
editor procedure to return (commit 54d01b5a0b), otherwise we end up with
a bad state in GIMP Protocol.
A proper solution to this will likely be to create a temp procedure from
metadata-editor to request it to close from another plug-in.
03398391e4