Localization still doesn't work, but this is normal (po-python is not
installed). I will later make the proper tests for this.

Other than this, it is a pretty simple port. It lost all particularities
and facilities of pygimp, but the fact that it now works similarly to
the C API is quite nice too.
It still uses the legacy API for plug-ins though and will have to be
ported further when the new API will be stable.

Also I still haven't figured out why we need to return the number of
returned values. With the proper annotations, an array length parameter
disappears in introspected Python (because it is useless as Python lists
know their length). But it would seem that this annotation doesn't work
the same for returned values, which is a bit sad as it creates ugly
redundancy.

It can be noted that I an going to move all Python plug-ins from
plug-ins/pygimp/plug-ins/ to plug-ins/python/. The whole pygimp/
subdirectory will actually be deleted eventually (I keep it around for
now as reference) as Python plug-in should not need to be considered
particularly from now on. They can just be considered as generic
executables.