Begin cleaning up QQmlValueTypeWrapper

Get rid of the objectType member. Instead of checking for it and then
static_cast'ing to the specific class, it's easier to use Managed's as<>
template cast.

Also elimiate the QVariant value member for ValueTypeWrapperCopy. In fact, the
entire class is folded into ValueTypeWrapper itself as d()->type has the one
copy of the data.

Change-Id: I0d9c794400095830540d313187623b98b686f4cd
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Further value type wrapper cleanups

Removed the QV8Engine * member in QQmlValueTypeWrapper.

Change-Id: I37cc131a3ff0279240b227c11a6b74bfc7bf4960
Reviewed-by: Lars Knoll <lars.knoll@digia.com>