Draft: Wip/jehan/signals gimp
Pushing the signal commits on a separate branch, as they have now been separated by the now-merged !112 (merged).
@mitch wanted to deal with libgimp objects life cycle in a different way, and it may mean that signals may be reimplemented in a different way too. Still putting these commits out there, in order not to leave these out-of-sight.
Edited by Jehan