Add AS::Metadata class used for parsing desktop and appstream metadata files

[grulja]

Same PR as #118, I just wasn't able to squash all the commits into one as there were some other commits among them.

[ximion]

Still looks good :-)
With all these new enum-to-string mappings, we need to be careful in future to adjust the Qt bindings as soon as the C bindings change.
I wonder if I could make some script to have the CI test for that...

Anyway, thanks for your contribution!
The next release will be massive, as I also want to get the YAML parser refactoring in, so it will likely happen in about two weeks, followed by faster releases again.