Implement desktops and layout regions

[Antikyth]

Layout regions are areas of the screen used for a tiling layout. Normally, that would be the whole screen, but they could be useful on an ultrawide, for example, where you might split the monitor in two using two layout regions. Of course, you might also want to just have the one layout region even on an ultrawide.

Probably the most useful way for layout regions to work is that they show different desktops, i.e. you can have two desktops open on the same monitor.