Introduce a wayland/x11 mutter hints protocol for monitor labels

[leolost2605]

Currently we do monitor labels quite awkard. First the display plug has to go via dbus to the daemon having to send color info, monitor index, label, x, y then the daemon spawns the labels and then gala looks at the window title and positions the label.

This can be made properly now by just using a wayland protocol/ setting the mutter hints on x11.

This will allow us to drop the GTK3 daemon with some more fine tuning to the menus so that's left to another PR.

This also includes some cleanup of the positioning logic for the shellclients by introducing a WindowPositioner which is used for both centered windows and monitor labels. That also fixes an issue that caused centered windows to sometimes not be centered.

Goes with elementary/pantheon-wayland#3 and elementary/switchboard-plug-display#401

[danirabbit]

Since we're in the stable release phase we can add new API but we can't break old API until the next major version bump

[leolost2605]

Ah right that is a thing oops 😅
This isn't very important just a better and slightly improved way to do it so it's obviously fine to wait
I think I'll factor out the WindowPositioner and the corresponding fix for centered windows though and prepare a separate PR for that (which, TBH, it probably should be anyway)