Draft: display: Add toggle to configure XWayland scaling

Mutter supports a mode now where XWayland clients can scale themselves up to an integer scale factor, instead of being scaled up by the compositor. While it works nicely with most x11 apps, it's making old x11 apps (which can't do scaling themselves) too small.

As a practical way to deal with this, let's expose a toggle for this in the display panel to change the behavior to let mutter scale xwayland clients (instead of clients scaling up themselves).

Hopefully we can at some point hide this toggle when no XWayland apps are installed anymore.

Marked as Draft because the design for the toggle isn't finished yet.

Goes together with mutter!3567 and gnome-settings-daemon!353.

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