expand use of Q_DECL_FINAL to seal types where appropriate

In general types that aren't designed to be subclassed should be systematically sealed.

Also using final helps the devirtualization machinery:
https://hubicka.blogspot.ch/2014/08/devirtualization-in-c-part-5-asking.html
(gcc -Wsuggest-final-types and -Wsuggest-final-methods)