Upstream logout button patch in temp/phosh
In postmarketOS edge, the only reason we have forked Phosh from Alpine is, that we have a patch that hides the logout button. This button isn't useful for us in the default single-user, single UI install (for multiple users/UIs, we expect people to follow this guide to customize their pmOS install).
We can get rid of the patch by making Phosh use gnome-session's CanSwitchUser D-Bus API (phosh#400).
When implementing this, let's make sure it behaves as expected with lightdm and tinydm.