Speed up lookups of imported scripts

The QQmlContextData stores the JS objects of imported scripts in a
QList<PersistentValue>. Instead of indexing into that list, this patch changes
ctxt->importedScripts to be a JavaScript array, that in the IR we can index via
subscript.

Change-Id: Ie2c35fb5294a20a0b7084bb51d19671a27195fec
Reviewed-by: Lars Knoll <lars.knoll@digia.com>