Fix qqmlvaluetypewrapper auto-tests

QQmlValueTypeWrapper stores its prototype in a global variable
(PersistentValue), which doesn't work when using multiple engines. Instead
this patch introduces the QML extensions structure to hold things like
prototypes for type extensions to JS that are specific to QML.

Change-Id: I41a71029edb6ce895eb80db7aa0809aab86b31db
Reviewed-by: Lars Knoll <lars.knoll@digia.com>