Track factory cleanup (#646)

* Don't need TrackFactory to make LabelTrack

* Don't need TrackFactory to make NoteTrack

* Don't need TrackFactory to make TimeTrack, or ZoomInfo in the factory

* Remove some forward declarations

* Rename TrackFactory as WaveTrackFactory, move it out of Track.cpp

include/audacity/ImporterInterface.h

@@ -143,7 +143,7 @@ public:
    virtual void SetStreamUsage(int streamID, bool use) = 0;
 
    // do the actual import, creating whatever tracks are necessary with
-   // the TrackFactory and calling the progress callback every iteration
+   // the WaveTrackFactory and calling the progress callback every iteration
    // through the importing loop
    virtual bool Import() = 0;
 };