Since Qt 5.11, QDoc uses Clang to parse C++ code for documentation.

As we have a combined documentation project under src/webengine that
covers also webenginewidgets, qmake does not pass the full set
of include paths required for Clang to parse also webenginewidgets
documentation. Fix this by adding the required include paths in
qtwebengine.qdocconf, and by using a custom module header, used
for doc builds, to pull in also the QtWebEngineWidgets module
header which Clang needs.

Task-number: QTBUG-66398
Change-Id: I1da547745c0c06762be216ae55724508b5d6a700
Reviewed-by: Jüri Valdmann <juri.valdmann@qt.io>
Reviewed-by: Kai Koehne <kai.koehne@qt.io>