Those fields have a slightly different meaning on each object that
extends DownloadableContribution and having them grouped in
DownloadableContribution only increase confusion in change of a
(very) tiny code reuse.
Moreover:
- the `readOnly` field has been renamed to `builtIn`
- predicates have been replaced by lambdas
- DownloadableContributionBuiltInAtTheBottomComparator has been replaced
with a singleton instance