Build "preview" debug service for the QML debugger infrastructure

The preview debug service will install a URL interceptor on any QML engine created by the application. The URL interceptor will change the schema of all URLs requested by the QML engine, so that the QML engine cannot load them locally anymore and will instead use the network access manager. The debug service will also install a special network access manager on all QML engines, which forwards the requests for the rewritten URLs through the debug connection to Qt Creator. Qt Creator will (hopefully) return the current state of the respective files, also through the debug connection.