The implementation of the undo/redo was simple but was letting the
complexity in the hand of the API users. In fact there was a need
to keep a prev state and and a current state, and to record only
previous states and not the current one. This was mostly working
(except for a redo bug discoverred while working on this new
implementation) but was tedious.

The new undo implementation is more complex but makes it far easier
to use. That is, modules can just record the current state and they
don't need to keep a previous state at all.

The undo history and map support has been reimplemented with this
new semantics and have been cleaned-up as a consequence.

This work has been highly motivated by the fact that we want to support
undo/redo for masks and that it was far too complex to do right with
the previous implementation.