Merge branch 'statistics-refactor'

* statistics-refactor:
  MetaFile::Track: prevent crash with the new statistics design
  Meta::NepomukTrack: organize header file, implement finishedPlaying()
  Meta::Statistics and subclasses: add docs and asserts wrt thread safety
  {Tag,Url}StatisticsStore: fix dates not read/saved due to wrong format used in sql
  No need to mention PersistentStatisticsStore explicitly now
  Implement Meta::Track::finishedPlaying(), ditch redundant implementations
  Meta::SqlTrack: use {begin,end}Update() for both Statistics and EditCapability
  Add set{First,Last}Played(), setPlayCount() to Meta::Statistics, ditch StatisticsCapability
  Adapt StatisticsProvider & co. to Meta::Statistics, rename
  Meta::Statistics: note about thread-safety
  Meta::Track: move statistics methods into statistics() interface

REVIEW: 106494
CCMAIL: Lucas Gomes <x8lucas8x@gmail.com>

Lucas, if you implement statistics in the Tomahawk Collections, you'll
have to adapt your code, just see the commit "Meta::Track: move
statistics methods into statistics() interface" how it is done.