Opens a dialog to create or edit a position of a person in an organisation,
e.g. a job Might be called from a person or an organisation editor.
Positions are currently stored as children of an existing person. So when
trying to add or edit a person to a given organisation, the "isBackward" flag
must be true. When creating a new job, this flag is optional and not used: it
can be deduced from the passed node type.