Prepare for QQmlPropertyCache to become available in QJSEngine

The cache is seemingly tied to QQmlEngine, but it isn't. A lot of
times a QQmlEngine parameter is unnecessarily dragged around and the
engine member is option as well as it turns out.

Change-Id: Iffd2a5046e9785249689ebfcbc8a0ad509f76aea
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Make QQmlPropertyCache available in QJSEngine

The QQmlEngine has two containers for property caches, one for QML types and
one for all-purpose meta-objects. The latter is rather useful and now being
moved to QJSEngine to be available there.

Change-Id: Ieab65c400b8a2e410e5f9eee6d603162dbb864d9
Reviewed-by: Lars Knoll <lars.knoll@digia.com>