Introduce QMDTrack as a superclass for QNetMDTrack and
QHiMDTrack to provide access to NetMD and HiMD tracks.
Introduce QMDDevice as a superclass for QNetMDDevice and
QHiMDDevice to provide access to NetMD and HiMD devices.
Introduce new class QMDTracks as a superclass for QNetMDTracks
and QHiMDTracks to provide access to NetMD and HiMD track lists.