inkex.py: build dict with selected nodes per sub-path per path

[su_v]

Follow-up report to:
Bug #171640: Pass list of selected nodes to script when in "Node edition"
https://bugs.launchpad.net/inkscape/+bug/171640

Add new method to Effect() in inkex.py to parse the new command line arguments passed from inkscape listing each selected node and build a dictionary of selected nodes per sub-path per path for ease of use in extensions (similar to getselected() and the dict self.selected).

Proposed diff attached (please do not commit without code review - feedback would be an appreciated bonus).

TODO:
- verify order of selected nodes per sub-path as passed from inkscape
  (IIRC currently, the order is not related to selection)
- if nodes are passed in selection order, either use lists for sub-paths,
  or track selection order separately (dicts in python are not ordered).

[su_v (suv-lp)]

Debug extension to test the new feature attached.

[jazzynico (jazzynico)]

Patch tested successfully on Windows XP, Inkscape trunk rev. 14648.

We have only a few extensions that could use that new feature, and I didn't find a easy way to convert them for now. But I'm sure it's going to be very useful for new extensions.

[jazzynico (jazzynico)]

Assigning to myself to be sure not to forget the report...

[su_v (suv-lp)]

Deleting the attached diffs and sample extension - I am revoking my pending patches provided in this bug tracker for issues with python-based extensions distributed with Inkscape.