Merge branch 'meta-observer-refactor'

* meta-observer-refactor:
  Meta::Observer: observing no longer prevents destruction
  Make Meta::Observer thread-safe
  Meta::Observer: use a bit of templating to remove ~150 lines of code
  Document Meta::Observer::subscribe() methods

CCing amarok-devel because this changes semantics of the core API
a bit.

CC: amarok-devel@kde.org