Rename the class to WebEngineContext and keep it as a ref-counted member
of pages instead. Also:
- Change the user-agent product to QtWebEngine
- Don't pass actual command line arguments to Chromium anymore
- Allow attaching to the event loop through Before/AfterRun instead of blocking

This layers things properly to be able to implement the UI in the
example application instead of directly in shell_qt.cpp.

This is still using global variables to allow the Shell platform
code to do callbacks to the API classes. This should go away once
we properly implemented a WebContentsDelegate.

Some more start-up fixes and a hack for test command line options for debugging multi-process startup