Near:
    - ZIP import
    - MS property set parsing

Longer:
    - More work on VBA directory parsing so that we can decompress
      source streams.
    - ZIP export

Future:
    - parse vba p-code directly to produce some other language.
    - move generalized meta data framework in here (see gsf/gsf-metadata-bag.h)
    - move biff utils here from gnumeric
