add a callback for layer changes

[RandomEtc]

To synchronize UI elements describing layers?

(updated title, was: "[layers-merge] do we need a layer-change callback?")

[tmcw]

I don't think so, unlike the purpose of the rest of the callbacks, layer changes are always started by user code, so the developer will always know & be able to call a callback themselves.

[RandomEtc]

Heh. I shouldn't have made the issue title a question :) It's not a show-stopper to leave this out, and it's an easy (non-bloating) addition in 1.x whenever someone needs it. Dropped from the milestone but left open.