[Alt-Tab] Overview of list scrolling settings.

also: Don't wrap around when scrolling in switcher list.
[Alt-Tab] Adjust close-to-edge-scroll behavior.
[Alt-Tab] Improve handling of closed window.
[Alt-Tab] Do away with a lot of Gnome-Shell stuff.
[Alt-Tab] Refactoring: remove some hopefully unused stuff.
[Alt-Tab] Try to scroll initial selection into view.