There are a couple of D-Bus services that are currently provided by
gnome-shell for which it makes sense to move them fully or partially
into separate processes:
- screen recording (performance)
- FDO notifications (security)
- Extensions (portalization)
Add some base classes and build system glue to take care of the
common boilerplate.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/547
