This fixes#5838 which was caused by an incomplete port
of this plug-in to the new introspection API.
For now we do keep the n_drawables parameter of save_ora until
issue #5312 is fixed. Not doing this would cause saving to fail.
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:
```
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),
])
```
Apart from porting, only code logics change is the whole
encode()/decode() code because it created a string vs bytes mess and the
zipfile API apparently didn't like to deal with bytes, even though the
docs say otherwise.
It's hard to test on my UTF-8 system, so please anyone working with
non-UTF-8 paths, we welcome reports if ORA load/save does not work
properly.
Other than this, load, save and load_thumb were all tested and working
properly so far.
Jacob Boerema