Change internal QML engine implementation so that it uses a new QDeclarativeExpression substitute

Currently, QDeclarativeExpression is used internally in the engine wherever a JavaScript expression is required (eg, dynamic functions, signal handlers, binding expressions). QDeclarativeExpression is a QObject-derived class, which has significant overhead. This task is to research whether an internal substitute for QDeclarativeExpression could be used which is not QObject-derived. QDeclarativeExpression must remain as-is for 3rd party developers, so some form of bridge must also be implemented.