Add support for gzip compressed plist files

Currently XBPS uses raw plist files, i.e they can be edited with your favourite editor because they are externalized as XML as specified in its DTD. That's ok, but files get bigger in size when there are tons of properties on them.

This blueprint when implemented will add support to read and write from/to compressed gzip files, by using proplib >= 0.4.1 with
the prop_{array,dictionary}_{in,ex}ternalize_{from,to}_zfile() funcs.