Clean up NepomukMeta constructors a bit

Phalgun,
* we prefer : m_field( initvalue ) initialization to { m_field =
initvalue }, because the first calls the constructor directly,
second calls the assignment operator.
* initializing class (non-primitive) types to their default values is
redundant, C++ guarantees their constructor is called. This applies
to QString, KShaterPtr and virtually all Qt class types.
* I wonder that KUrl m_playableUrl = NULL; even compiles, don't use
NULL in Amarok code at all.

Also removed unnecessary crippling of NepomukTrack::discNumber() and
a comment in NepomukArtist constructor that was invalid. (or I didn't
get it).