windowManager: Allow switching workspaces with super-scroll

windowManager: Allow switching workspaces with super-scroll

So far, we couldn't allow workspace scrolling outside the overview
because scroll events were always sent to clients. However mutter
was changed recently to pass on scroll events when the mouse button
modifier (usually super) is pressed, which allows us to enable the
same workspace scrolling as in the overview now.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1612>