Add no-op getter/setter methods

This prevents having to implement all the methods in subclasses
if only specific ones are needed (see FieldChart).
Change field's default node type to Condition

There is no built-in support for adding multiple conditions for the
same node, so until then only a single condition is necessary.

Fix bug thought to be fixed in 9b6e034e
Setup bi-directional observing between control and context

Update FieldChart to match recent API changes in Control class