Packs used images in sorted order into theme .zip files

The essence of this module is contained within [[solenv/bin/pack_images.py]].
This script takes several sources of information:

* the list of images used in every .src file that is compiled
* the configuration XML describing every command that is used
* [[packimages/pack/image-sort.lst]] which provides a profile based
  ordering of images.

It then tries to pack the images together into the .zip file into an
order that matches / is similar to that which we want at run-time, ie.
an entire toolbar's images should (ideally) be next to each other in a
nice linear run, to reduce LibreOffice's working set, and improve
startup performance: we require all these images to get paged in on
start.