QmlJS: Improve completion and hints for functions.

* FunctionValues know about optional arguments (for builtins)
* ASTFunctionValues only report themselves as variadic if they
  use the 'arguments' array.
* Function argument hint shows optional args and variadic.
* Completion automatically adds parentheses.

Change-Id: Ib2598600ff8b1ce8c5de3bcabd24a3e171ff3a57
Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>