This means that all functions which were returning or taking as
parameter an image id (as gint32) are now taking a GimpImage object
instead.
The PDB is still passing around an id only over the wire. But we create
an object for plug-ins to work on.

This is quite a huge API break, but is probably the best bet for the
future quality. It will make nicer API instrospection (and nicer API in
binding), will fix the issues with pspec on GimpImageID in Python
bindings (which makes the current Python API unusable as soon as we need
to work on images, which is most of our plug-ins!), etc.
Also it will allow to use signals on images, which will be a great asset
when we will finally have bi-directionnal communications (i.e. plug-ins
would be able to connect to image changes, destructions, and whatnot).