UI freeze break request for Mutter: Style X11 frames using libadwaita on GNOME

I'd like to request a UI freeze break request for this merge request: GNOME/mutter!3981 (merged)

What it does is, on GNOME-based desktops, it loads libadwaita and uses it styling for X11 frames around windows. In practice the difference is that X11 frames are a little whiter (or less dark on dark mode), and shadows are slightly bigger. There is a video in the MR with the result of the merge request.

I don't expect it to be a controversial change, visually the difference is fairly small.