Get Image -> Rotate 90/270 working with layers

Unpleasant project; glad to have it over.
Get Image -> Rotate arbitrary working with layers

This is the big one!  To my knowledge, no other free photo editor
(including biggies like GIMP and Paint.NET) allow arbitrary rotation of
layered images.  They all require you to flatten the image first.

But we are better than that.  ;)  Large images might take some time to
rotate, but the results will be perfectly aligned (and by perfectly I
mean with sub-pixel accuracy!).

Also, I got live previews working again.