With the version 4 of the screencast portal, it is now possible
to request and use restore tokens [1] so that apps can restore a
previously configured screencast session without user interaction.
Add the corresponding code to linux-capture's PipeWire source.
Store the restore token in the source data, since each restore
token corresponds to an OBS source, and use it as soon as we try
to create a new session. Implement the obs_source_info.save vfunc,
and save the restore token when it's received by the Start()
response using obs_source_save().
[1] https://github.com/flatpak/xdg-desktop-portal/pull/638
26f1bfd05f