When a new playlist's mediaObject is set, the content is cleared and the
mediaRemoved() signal is emitted without a former
mediaAboutToBeRemoved(). This is an issue for QAbstractItemModel
implementations, like the coming QDeclarativePlaylist, which call
beginInsertRow() and endInsertRows() in the respective signal handlers.
Change-Id: I7ec512ff2736e92858df94d9479741e05162e1f0
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>