Export thread control to generated wrappers

Since the default OpenCV distribution comes precompiled with threading support enabled, it is important to have the ability to limit / disable the option in run time. Currently the relevant functions are available in C++ only, and thus force the user to have it on when being used via C/Python/...

This patch exports the functions setNumThreads, getNumThreads, getThreadNum.

The issue filed in opencv code is:
http://code.opencv.org/issues/4064