Cache class info to speed up connection

As of now, QMetaObjectPublisher always re-populates the JSON Object describing an object when a client connects. This process is rather expensive and hits hard when exporting a large number of objects. In our software, QWebChannel is the main remoting interface to a rather complex application (exporting up to about 50 objects) running on limited hardware (i.MX53 based).
When connecting to the WebChannel server it can easily take up to 15 seconds before the whole initialization message is built and sent.

This could easily be fixed by building the class info once (including everything except current property values) and storing it in a cache keyed by the QMetaObject.