2202: Behavior of read-only maps:
- treat read-only maps (chmod -w *.mm) and documentation maps the same
- allow the user to edit read-only maps, but not save them (error popup)
- don't display '*' for modified read-only maps
- TODO: handling of modified read-only maps on close
- TODO: read-only maps + scripting API
2202: Behavior of read-only maps: Fix handling of read-only modified maps on close
2202: Behavior of read-only maps: Error box when trying to save to
read-only file (instead a mere log msg which can easily be overlooked!)
2202: Behavior of read-only maps: Fix close behavior
(also clean up and make sure that isSaved()==true for new docu maps)