Add study notes

The user should have the possibility to write comments or study notes which are not based on a verse list like the current "Personal Commentary".
Instead he should be free to write whatever he likes and structure it however he likes.
This gives much of a word processing functionality using NSTextView.
Study notes are by default stored in the "~/Library/Application Support/MacSword/StudyNotes" folder and can of course be exported.
They are managed and listed in the left side bar.