Re-design "Add dependency"-frame
New "Add dependency" window should contain 2 combo boxes for courses and the already existant check boxes.
When drawing a new connection in graph view, both combo boxes will be pre-selected by the graphically chosen courses. Additionally it should be possible to call the window from the context menu of the list view. In this case only the dependet course will be preselected and the user must chose the course it is dependent on.