If a sandboxed app requests access to some system resource (camera,
microphone, location), the portal frontend needs to ask the user
for permission. In GNOME, we want this to be a system modal dialog,
so provide an org.freedesktop.impl.portal.access implementation
that exposes a generic system modal permission dialog on the bus.
https://bugzilla.gnome.org/show_bug.cgi?id=768669
