gimppdb: Allow more easy bindable API

Plug-ins that work from different bindings probably want to use their own list-type to specify arguments, rather than working with a more cumbersome GimpValueArray.

This new API should make it less verbose. For example, in Python:

args = Gimp.ValueArray.new(5)
args.insert(0, GObject.Value(Gimp.RunMode, Gimp.RunMode.NONINTERACTIVE))
args.insert(1, GObject.Value(Gimp.Image, image))
args.insert(2, GObject.Value(Gimp.Drawable, mask))
args.insert(3, GObject.Value(GObject.TYPE_INT, int(time.time())))
args.insert(4, GObject.Value(GObject.TYPE_DOUBLE, turbulence))
Gimp.get_pdb().run_procedure('plug-in-plasma', args)

becomes

Gimp.get_pdb().run_procedure('plug-in-plasma', [
    GObject.Value(Gimp.RunMode, Gimp.RunMode.NONINTERACTIVE),
    GObject.Value(Gimp.Image, image),
    GObject.Value(Gimp.Drawable, mask),
    GObject.Value(GObject.TYPE_INT, int(time.time())),
    GObject.Value(GObject.TYPE_DOUBLE, turbulence),
])