Add a way to call native functions from javascript

The idea is to be able to interact somewhat directly with V8 and to be able to expose C++ functions to JavaScript.

My understanding is that the native C++ code must be in QtWebEngineProcess.exe.
One possible solution is to have QtWebEngineProcess.exe load user plugins that contain the native C++ code.

To make this useful likely the plugins need to communicate through IPC with the main application. Maybe it can be handled "Qt side" which would require adding a "sendIPCMessage()" function to QWebEnginePage. Or it could be left to the users to do their own IPC, but it would require sharing an ID of some sort so that they can indentify which process is associated to which QWebEnginePage (maybe the existing renderProcessPid is enough?).