Be able to edit the source file in the main interface about key

[WolframPfeifer]

Thanks for the suggestion. Unfortunately, this feature is not easy to implement. At the moment, the source view on the right side shows the source code corresponding to the currently selected proof (basically a shadow copy created at the moment the proof was started). If it was possible to edit the content, then the two view would be inconsistent. It is not clear at all how the feature would look like. In addition, we would have to maintain our own editor, which is much harder to implement than "just" a read-only view of the code (you have to implement and support various edit operations, syntax highlighting during editing, ...). For these reasons, we decided to keep the view read-only at the moment.