Make New lesson button work.
Delete all <no lesson> stuff in favor of adding vocabulary without a lesson to "Default lesson". This should still be compatible with all old files and old versions will still be able to read the files.

Because the <no lesson> is away KVTLessonModel no longer needs an internal lesson list but is again directly using the data from m_doc.

KVTLessonView now has a slot to add a new lesson. The old way still works (property dialog) but currently does not update the lessonview.