Add relationship between dialogs (if one exists)

[Boris Rizov]

Some i-doit dialogs have a relationship, such as the model manufacturer and the model title. When using the API it's impossible to tell which item belongs to which manufacturer or vice versa, which leads to hard coding of some values instead of dynamically fetching them. It would be great if the API could accommodate such functionality (i believe the i-doit api class needs a little tweak to make this work).