Switch workspaces on pgUp/pgDown

[timbertson]

Not sure if you're open to including this, but it seems pretty closely related to this extension. I was going to write it as its own extension but hadn't got around to it, and this extension provides a good base.

So I won't be offended if you say no, but I will be happy if you say yes ;)

[tanwald]

Thanks for the contribution. I will merge it soon and give credit to you. Maybe switch_workspace should become a member of worspacesView. Right now I'm to tired...

[timbertson]

Cheers - yeah, I wondered about that too. I was going for less
intrusive, but I've seen that method scattered throughout multiple
extensions already. If this is going to be in gnome-shell proper I'd
definitely add it to WorkspaceView, but since it's an extension I'm
not sure there's much to be gained by putting it on a class you don't
own.