EngineController: ditch canDecode()

MetaFile::Track::isTrack() is partial replacement. Existing 3 calls to
canDecode() were in fact all related to MetaFile classes, so move the
method there, simplify it not to query phonon at all (and document it
can return false positives).

As a consequence, we show all audio and video files in file browser and
in other places, even if they wouldn't be playable by the current
phonon back-end. This is arguably a cleaner approach and at least lets
users discover where the error is.

Works quite well for me and prevents failures in many tests. This
change is propelled by Bart's and Ralf's legitimate comments on