Operator to cycle through an Area's previous Editors

Probably an anti-feature, unwanted, or dumb. But still seems interesting to ponder.

Currently you can change any area from one Editor type to another, usually by using the Editor menu at the header's top-left. Our keymap also allows directly changing to specific editors by pressing Shift + Fx, ie Shift-F5 for 3D Viewport.

But what if you could just click "back" to get back to the previous one? With this patch you can. Ctrl-F1 gives you the previous editor, while Ctrl F2 moves you to the next in the list.

This could open up new ways of using Blender. You could choose to use less - but larger editors - and just jump between them as if they were tabs within an area. It also fits nicely with docking - D16648: WIP: Harmonious Area Docking, Joining & Splitting - because that allows dragging one area onto another, and this allows access to both.

The code might be a bit suspect, mostly just copied from portions of ED_area_newspace. Would need close review from someone who knows this area well.