This listens to org.freedesktop.portal.Flatpak and lets flatpak
sandboxes do flatpak specific things.
Initially this only allows access to "Spawn", which lets you start a
new copy of the current or latest version of the calling app,
optionally with sandboxing.
This allows the app to re-start itself with the latest version after
an update, and the sandboxing is useful for apps that want to manually
sandbox part of themselves.
You can also expose one or more subdirectories of
~/.var/app/$appid/sandbox/ to the app, read-write or read-only. This is
useful to communicate with the sandbox.
This is now in xdg-desktop-portal. We keep a version of the document
portal dbus XML so that we avoid weird build dependencies.
Flatpak itself is technically not dependent on the document portal,
but it is very much recommended that you use it.
Closes: #1398
Approved by: alexlarsson
