In preparation for caching Contexts.

Reviewed-by: Erik Verbruggen

* Context::build was potentially dangerous. Link and ScopeBuilder
  went out of scope before their results stopped being used.
* ScopeBuilder: If no function scope object was found, don't add it.

Reviewed-by: Erik Verbruggen

Task-number: QTCREATORBUG-1413
Reviewed-by: Erik Verbruggen

Task-number: QTCREATORBUG-1388
Reviewed-by: Roberto Raggi

Use ScopeBuilder.push(...) for the same functionality.

Reviewed-by: Erik Verbruggen

Allows completion of local variable and function names.

Task-number: QTCREATORBUG-942
Reviewed-by: Roberto Raggi

* The "Script" type is now implicitly available without imports.
* No errors for bindings inside a Connections element.

Reviewed-by: Roberto Raggi

By always setting the scope object, even if it is identical to the root
object.

Done-with: Erik Verbruggen