Remove instances of manual manifest parsing

[vgmoose]

There are a few occasions now where hb-appstore consults libget's store of manifests files to go through the files and perform some checks. It would be better if libget had something like a Manifest object and a way to retrieve a list of all files, with a certain status, matching a certain regex.

One instance of this is theme installing, where we need to fetch all paths of `.nxtheme files from the installed manifest.