Added possibility to select between which windows to switch (all or from current workspace only)

Gnome likes to change the Alt-Tab behaviour with every version. Well, I liked the behaviour most where 
you could switch primarily between the windows from the current workspace but also continue
(with Alt-Tab) to windows from the other workspaces. As gnome 3.7 doesn't provide this possibility
anymore, I added it to the Alt-Tab extension that I'm using now.

I think others might like this, too.