Adapt StatisticsProvider & co. to Meta::Statistics, rename

...and use directly in Track implementations, which saves some code.
Also move all to core-impl/suppor - it is just an implementation
detail.

Also modify PersistentStatisticsStore (legacy StatisticsProvider) to
call notifyObservers() as appropriate, which needs some friend
declarations in Meta.h. Use new Observer functionality to track
linked tracks without holding a reference to them. (to avoid circular
reference counting)