qt: don't inherit from QObject for MLBaseModel children
QObject is a large object and inheriting from it is expensive. One of the advantages of models is we don't need their children be QObject. So remove the inheritance from QObject whenever not required. This MR deals with MLAlbumTrack (see MLAlbumTrackModel), MLAlbum (MLAlbumModel) and MLAlrtist (MLArtistModel).