MemoryMeta: factor some logic MapAdder -> Track, prevent circular refcounts

We cannot easily store KSharedPtr to tracks, because it creates reference
counting cycle: MemoryMeta::Track::m_album -> MemoryMeta::Album::tracks() ->
MemoryMeta::Track. We therefore store plain pointers and rely on
MemoryMeta::Track to notify when it is destroyed.

ChangeLog is updated with entry for USB Mass Storage collection where
this change is user-visible. (well, theoretically) :-)

DIGEST: optimisation: fix memory leaks due to circular references