Implement a first rudimentary dive-editing command. The main code
resides in a base class Command::Edit, which calls virtual functions
to read / set the fields and extract the field name.
Implement an example: editing of dive notes.
This dose not yet update the UI on undo / redo.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
