QmlJS: Fix a performance problem.

In order to determine all components that instantiate a given component,
Bind::usesQmlPrototype was called on each document in the snapshot.
That, in turn, had to iterate over all object definitions inside the
document, resolving their prototype references.

All in all, it lead to a very large amount of fairly expensive lookups
that were slowing down Creator considerably.

Reviewed-by: Erik Verbruggen