Window decorations key on Settings portal

### Problem

On Linux, window decorations are a mess, especially when it comes to CSDs. Pantheon using Mutter means it is in the same boat as GNOME, where the window decorations are app-drawn and aren't "consistent".

Some apps will follow the GTK `button-layout` preference to a T (even duplicating buttons if desired), some will manipulate it how they like and only partially follow it, and others won't follow it all.

I'd like to address this by "upstreaming" the `button-layout` preference, and I imagine Elementary would benefit from this, as they did with the `color-scheme` preference.

### Proposal

Pantheon adds a Window decorations key to the Settings portal, in favor or toolkit-specific implementations (such as GTK).

The art of *drawing* the decorations, aka the SSD vs CSD argument, is out of scope here. This is just general preferences around how to draw them. With the xdg-decoration Wayland protocol and this portal preference, the UX is more consistent, and more desktop-agnostic.

### Prior Art (Optional)

See https://github.com/flatpak/xdg-desktop-portal/pull/996, heavily based off of how GTK handles it.