new API for StereoMatchers

rewritten StereoBM in C++ (also converted TBB's parallel_for to universal parallel_for), refactored StereoSGBM too. Now we use the same Algorithm-based class StereoMatcher as universal base class for stereo correspondence algorithms. The actual classes are not exposed in headers.

this pull request is primarily for testing. However, it can safely be merged if buildbot says "ok". The only thing that would have to be done after that is extending calib3d_init.cpp to add stereo algorithms initialization into initModule_calib3d().