Meta::Track: move statistics methods into statistics() interface

* introduce Meta::Statistics, an interface to statistics-related
methods. Memory manager using KSharedPtr.
* remove (set)score(), (set)rating(), first/lastPlayed(), playCount()
from Meta::Track and move it to Meta::Statistics
* Because Meta::Statistics uses KSharedPtrs, existing track
implementations can just inherit Statistics and return this; in
Track::Statistics()
* Existing Track subclasses with dummy implementations of (set)score(),
(set)rating(), first/lastPlayed(), playCount() have it removed.
* Existing Track subclasses with meaningful implementations of
(set)score(), (set)rating(), first/lastPlayed(), playCount() turned
into Meta::Statistics and statistics() method is added for them.

Next steps:
* Make StatisticsProvider an implementation of Meta::Statistics, which
will in turn immensely simplify all tracks that use them.
* Add setPlaycount(), setFirst/LastPlayed() to Meta::Statistics and
ditch StatisticsCapability.
* Implement trackPlayed() in Meta::Track, ditch custom implementations.

Heavily based on Ralf's idea and patch in
https://git.reviewboard.kde.org/r/106276/ it is just attribute vs. the
subclass pattern + some minor details like notifyObservers() in
StatisticsStore, circular refcounting avoidance etc.

I want to deal with other track Capabilities in similar way in future,
the next one being EditCapability: I'd like to introduce Meta::Editable
and Meta::Track::editable() and rename {begin,end}MetaDataUpdate() to
just {begin,end}Update() to share even greater portion of code in Track
implementations.