Support additional descriptions on Field

[aalmiray]

Similar to dlsc-software-consulting-gmbh/PreferencesFX#23 sometimes you'd like additional text to be displayed under a Field's name or it's renderer. This text may be used as a hint to the user to let them know what data may be entered as the Field's value.

It may be worth using Node as a type instead of just String as this would allow for additional elements (such as icons) to be embedded in the description.

I propose to add two more properties to Field: labelDescription(Node) and valueDescription(Node)/fieldDescription(Node).

[dlemmermann]

Sounds good to me. As long as it does not break the current API obviously.

[aalmiray]

Fixed.