gnome-vfs offered a way to monitor the association between mime types
and default handlers for changes, with the GnomeVFSMIMEMonitor object.
GIO does not offer a replacement for this functionality at this time,
since we have not found a compelling use case where
GnomeVFSMIMEMonitor was used. If you think you have such a use
case, please report it at
https://gitlab.gnome.org/GNOME/glib/issues/new.