There are 2 TODOs to take care of here. First, there is no GUI yet for
the interactive mode which will just use the default values.
Second, the color argument is not working yet because I had issues
passing a GimpRGB as argument (ideally I should create a GimpParamRGB
with gimp_param_spec_rgb(), but since we still have the pygobject bugs
about manipulating GParamSpec data directly, I can't).

Anyway it works with default values, which is already a good first step.
:-)