Introduce a wayland/x11 mutter hints protocol for monitor labels

[leolost2605]

Currently we do monitor labels quite awkardly. 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.

Goes with elementary/pantheon-wayland#3 and elementary/settings-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)

[danirabbit]

@leolost2605 is it possible to add the new API without breaking the old API so we can move forward?

[leolost2605]

@danirabbit finally got around to updating this so it should be ready without breaking the current API :)