Enable gadget wrapping for custom value types

Enable gadget wrapping for custom value types

[ChangeLog][QtQml] Custom C++ value types annotated with Q_GADGET are now fully
accessible in the QML and QJSEngine JavaScript environment.

QJSEngine::toScriptValue can be used for injection and fromScriptValue to
extraction. The QML "built-in" gadget wrappers for QPoint and the gui types are
not exposed this way, toScriptValue(point) will still return an opaque QVariant
wrapper. We could expose the core types right away, but then we would be
lacking an API to enable use of the Gui types that are registered in QtQuick.

It would be better to make the core types in qtbase gadgets and thus enable
them without the need for hooks and init functions to be called by the user.

Task-number: QTBUG-29769
Change-Id: I8179cd599bdc1209ff61cfdbdda419cb400296bb
Reviewed-by: Lars Knoll <lars.knoll@digia.com>