Rough port of the documentation to QtWebEngine

This is a gross mechanical modification of the documentation, along with build
system bindings to allow it to generate. This should allow doing iterative
improvements to the documentation from now on.

This fixes project-related qdoc warnings but we still need to do some serious
work to get the documentation better fitting QtWebEngine. All the documentation
is ported to match the current state of our headers (without trying to adapt
to modified APIs yet) and we should clear the part of the documentation that we
don't need at the same time that we clean up our public headers.

Change-Id: I6fb4e10e8b4c1c53be7bc7c581286248ac04d4da
Reviewed-by: Andras Becsi <andras.becsi@digia.com>