Phonon: add a function that can determent if a file is playable.

It would be nice to be able to ask the phonon interface if a certain binary file is
"playable", whether it be video or audio.

There is no obvious way to get a mime type from a file and when one use
the Phonon::BackendCapabilities::availableMimeTypes() on a system for
example, it comes back with 38 possibilities.