Changed the Method 'save' of the literature dbobject. The ref counter of the old entry gets now decreased.
Had to split the function save to a second function edit. Save only saves the object. Edit checks if there are refs and so on.
Implemented the edit form. Logic coming soon.
Added edit form for literature entries in a course. Logic isn't fully functional. The coverpath isn't supported (cover will be deleted) and there is no possibility to add a new link.