... for doxygen. Change logic for known empty keywords to be replaced
by an empty string.

Task-number: QTCREATORBUG-1854

To catch cases of invalid code like this:
struct A : B {};
struct B : A {};

Reviewed-by: Roberto Raggi

Task-number: QTCREATORBUG-1896

Using more *::Constants::ICON_* where it makes sense and
wrapping the file names into QLatin1String where they were
missing.

The increased usage of the ICON constants needed a few more
cross plugin includes of *constants.h, here and there.
I think that it is OK, since the dependencies were alredy there
icon resource wise.

For answers to questions about where to insert a snippet/chunk of C++ code. Ok,
currently it will only find the One And Only place to insert method
declarations in classes, and it will need some tuning.

The old integration option is now used to switch to extended descriptions.