The user needs an interface to edit _nosynch and _onlysynch files.
Can be something very simple, like an text editor or something more sofisticated
like a table with "Add" button, where the user can add entries and select if
those entries are a "Regexp" or not. In the cas it is a regexp, the system can
validate the syntax before saving.
This interface must show all places that this .nosynch or .onlysynch can be
saved (maybe showing if the file already exists or not), so the user can choose
the one that fits better his needs.