Draft: Add support for systemd-homed

This is my branch for working on systemd-homed integration w/ GNOME OS

Depends on !2661

If you want to try this branch, you'll need to use BuildStream's workspaces feature to apply some patches. As a quick refresher, here's how to do that: bst workspace open ELEMENT --directory /path/to/somewhere. Then you can apply the patches using git. Here's what you need to apply:

• To the element core-deps/systemd-base.bst
  • Make logind handle secure-locking
  • bugfix (not strictly required)
  • Allow users to change their own settings (not used by GUI, but convenient on the CLI)
• core-deps/accountsservice.bst
  • comprehensive homed support (which contains basic homed support)
• core/gnome-initial-setup.bst
  • support for homed
• core/gnome-control-center.bst
  • support for homed
  • bugfix (not strictly required)
• core/gdm.bst
  • support for logind secure lock
• core/gnome-shell.bst
  • support for logind secure lock

Once the patches are applied, run utils/run-secure-vm.sh --debug --reset. Eventually the VM will come up and you can test it all

Part of #655