For now this doesn't mean much, but it will help when we need data
exclusive from the metadata, such as addon id and mod provider.
Also removes the metadata when the mod is deleted, and make the Mod.h
file a little more pleasing to look at :)
Let's move off our custom QuaZip. In the olden times we needed the
custom version of QuaZip, as it was basically unmaintained and on
SourceForge (eww). But nowadays it's maintained and on GitHub. See
new GitHub page: https://github.com/stachenov/quazip
