Allow manual addition of cheese connections

When one or more text files are open, provide an easy way to connect text segments with a string of cheese indicating some relationship between them. Maintain connections when the file is edited, and lines are added or removed. Allow editing or deleting of existing connections. Save these manually-created connections in the project YAML file. Possibly include some context-sensitive method of reconnecting the appropriate line(s) even when the file(s) are edited in an external application.