QJSEngine/QQmlEngine: Support calling C++ static member functions, namespaced functions, and/or standalone functions

QScriptable allows us to expose C++ classes with static and non-static methods to JavaScript.

var obj = new Class();
obj.nonStaticMethod();
Class.staticMethod();

However, with QQmlEngine, we can only call C++ instance member functions from JavaScript.

Current related deficiencies:

• Q_INVOKABLE does not work with static member functions
• Q_INVOKABLE does not work with Q_NAMESPACE
• There is no equivalent of QScriptEngine::newFunction (QTBUG-45246 )
• QQmlEngine does not let us instantiate C++ objects using JavaScript `new` (QTBUG-103461)