Improvements to the completion of identifiers

[richardpawson]

1) Instead of being managed at field level, identifier completions should be the responsibility of the appropriate ParseNode.

• In the first instance this would be made to work only for the last parse node in the field i.e. when entering the field sequentially.
• Maybe it could be extended to work when editing text within a field, but I suspect this will be both difficult to implement, and less satisfactory from the user's p.o.v.

2) Make it more context sensitive, especially after a . (dot-methods, properties, qualifiers)

[richardpawson]

Superseded by auto-complete