Add support for argument guessing in constructor completions

[eric-milles]

Below are the completion proposals for a typical group of constructors.

When selecting one of the proposals with parameters, Groovy does not offer argument guesses like Java does:

NOTE: ConstructorCompletionProcessor, GroovyCompletionProposalComputer, et al. do not fill in the extended context required by GroovyJavaMethodCompletionProposal to enable argument guessing. StatementAndExpressionCompletionProcessor handles this for the method completion case.