Daemon: Implement monitor labels

[leolost2605]

Taken from #1840 (was easier than rebase :))

Fixes #1823

For the switchboard side see elementary/switchboard-plug-display#372

[lenemter]

I'm not sure if window title is a good way to do this. I remember #1427 was rejected for this reason. @tintou how should gala identify gala-daemon windows?

[leolost2605]

Yeah I wasn't sure either but I think the difference to #1427 is that we already know for sure that it's a daemon window and only differentiate between different types of daemon windows 🤷

[tintou]

Note that handle_daemon_window won't be called on X11 so we need another wrapper for X11, maybe by just ensuring that ("%" + uint64.FORMAT).format (window.get_pid ()) == subprocess.get_identifier ().
I think that this is good enough for a first iteration, ideally the daemon should communicate each type of window with a dedicated DBus signal, but that would probably be better to iterate on it so that we can move faster.

[leolost2605]

Note that handle_daemon_window won't be called on X11 so we need another wrapper for X11, maybe by just ensuring that ("%" + uint64.FORMAT).format (window.get_pid ()) == subprocess.get_identifier ().

Since the daemon is still GTK3 I've just used the GTK3 methods for positioning etc. for X11