…d manager refactor

This kind of ballooned out of scope, but I implemented some much
needed fixes for the download manager.

First off, I cleanup the Downloadable trait, there was some
duplication of function.

Second, I refactored the "validate" into the GameDownloadAgent,
which calls a 'validate_chunk_logic' yada, same structure as
downloading.

Third, I fixed the progress and validation issues.

Fourth, I added game scanning