Review/clean-up Field class

[corranwebster]

Once #1020 is merged the Field class and the associated testing code have become very thin and have no toolkit-specific code in them.

We should either:

• simplify things (eg. move the implementation of Field out of the toolkit specific code, remove empty classes)
• find some new toolkit-dependent functionality that really belongs on a generic Field and implement it
• (in the case of testing code) see if there are common things in the subclasses that belong in the mixin.