Add eager completion of children variables in expressions

[4ian]

In addition to traditional completions:

Also display completions for children variables if the exact name of the variable is entered.

This means a full structure can be browsed by choosing the right children, without entering a dot manually.

• Also avoid listing children with names that can't be used in expressions
  • List them with [""] syntax.
• Parser fixes
  • Improve error messages so that Variable.. is not accepted.
  • Fix Variable.["test"] that should not be accepted