Completely reimplemented the Audio track splitting. Sadly the old implementation crashed on a regular basis and had some usability quirks. A complete reimplementation was much easier than understanding Sharan's code. Also the new code is much more compact and does not break the AudioEditorWidget API. Thus, the Source editor dialog works properly again.

Sorry, Sharan, but I had to make it work and the only way for me was to redo it.