Support thread-debugging tools in Qt

There are tools that help detect race conditions in multi-threaded code. So far I have tried Valgrind's helgrind, DRD, Clang's TSAN  (thread sanitizer), Intel Inspector (tested on Windows by manordheim).

All the tools were full of false positives, because they don't understand Qt's locking primitives. TSAN didn't even reach that point, it crashed earlier.

This task is about investigating which tool can support Qt and what it takes for that.

Adding such support will increase quality of both Qt and its users:

• Qt will be able to integrate the tool into CI and catch race conditions and other similar problems early
• Users of Qt will also be able to debug their multithreaded applications using such tools.