Add button-placement key to Settings portal

[TheEvilSkeleton]

This is a succession of #996. I'm also going to look into providing an implementation in GTK and xdg-desktop-portal-gnome.

Closes #956

[orowith2os]

Can you include the exact reverse of this example, just to be clear on how it works?

      - `(["close","minimize","maximize"][])` would result in the close,
        minimize, and maximize buttons being drawn on the leading side of the
        window, in that order; there will be no buttons on the trailing side
        of the window.

I.e., all buttons on the leading (right) side of the window.

[orowith2os]

I did an implementation for xdp-cosmic here: pop-os/xdg-desktop-portal-cosmic#199

[swick]

Still requires:

• at least one implementation in a backend
• at least one implementation in a toolkit (GTK is on board with this portal feature)