Implement new build system as discussed on IRC.

FindPhonon is to be held in every consumer (it being very small thus unlikely to change). Via pkg-config Phonon at install time sets its buildsystem directory, where it stores all cmake files of value. When a consumer then uses FindPhonon it will ask pkg-config for the buildsystem dir and start PhononInternal from there, which then takes care of finding Phonon and setting common magic (such as compiler flags).