Qt should use C++11 features (with macro guards) in existing APIs.

If Qt starts using, for example, the final specifier on classes that are not intended to be inherited or on classes that are not intended to be overridden, then for 3rd parties doing those newly forbidden things (while already using c++11 in their build flags) it would be a source incompatible change.

We should evaluate if any scenarios like that are real-world enough, and see if any action can be taken.