Implement QWebEnginePage::toHtml and toPlainText

Those methods are now made asynchronous and need to be given a
callback to handle the result.

Update the code in the browser and fancybrowser examples using
std::bind when using C++11 or tr1::bind with C++03 (which should be
available with compilers on platforms that we support).

Add a (currently failing) earlyToHtml test to make sure that an empty
page doesn't crash because of a possibly incomplete attachment of
the QtRenderViewObserver.

Change-Id: I3ab7cb6f25b91b584dd80df5e4e9ad1e3214348e
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>