Make lessons a list again.
QMap is too flexible. Users will kill us for changing lesson order. The flexibility of the map makes the lesson model in KVocTrain much harder to implement.

Moved the inQuery flag into the lesson class.

Fixed readers and writers. I could not verify if the keduvocvokabelnreader.cpp works.

The first lesson has 0 as index now.
A default lesson is created according to Jeremys plans, but within the document class itself, after reading the doc, so all readers profit from it.
KEduVocExpression is not initialized to lesson=-1 as default.
The old reader maps lesson 1 to 0.
Splitting and deleting lessons seems to work.

Moving lessons is not implemented (this will become interesting, when drag and drop for the lesson model is enabled).
TODO: When adding an expression, the document can automatically add it to the lesson.

Right now we call doc->lesson.add(Exp) and doc->addExp(lesson). The later should suffice.
Fixes in KVocTrain (many +-1 are no longer needed).