wayland: Only load HiddenPreviews options in X11
HiddenPreviews was a hack to not inform X11 that windows were minimised, so that their previews kept working.
On wayland the user facing setting is not visible but the value is loaded which is a bad combination.
(cherry picked from commit 1c3a987d)
2cb8a700 wayland: Only load HiddenPreviews options in X11