Support virtual desktops

[andydotxyz]

Let's make better use of the space we have :)

Needs a pager too (separate issue at #28) and some handy keyboard shortcuts.

Should they be per-screen or refer to all-screens when shifting?

[Jacalz]

Though it will be significantly harder, we really need touchpad gestures for this as well. I can’t live without three or four (Gnome uses three, Windows uses four) finger swipe to switch between virtual desktops 🙂