refactor factorization a tad and de-duplicate the lookup logic in settings

previously we had the (almost) same logic between libphonon and settings,
since the settings are now in-tree we can easily remove the duplication
by exporting a new (privateish) finder on the factory that is used in the
settings app and the actual factorization

also extend descriptor a bit to more easily match functional requirements
of libphonon factorization. notably it now has a pluginname for mapping
to legacy PHONON_BACKEND env override and a weight which is the configured
preference