wayland: Explicitly handle classic mode for now
wayland: Explicitly handle classic mode for now

There are plans to add session-dependent defaults to GSettings
(based on the newly standardized XDG_CURRENT_DESKTOP); until
then, the WM uses a different schema for its button-layout
setting in classic mode. So for the time being, do the same
and pick the alternative schema when XDG_CURRENT_DESKTOP
indicates that we are in a classic session.
(It's not pretty, but hopefully won't be with us for too long ...)

https://bugzilla.gnome.org/show_bug.cgi?id=731273

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