Looks like we don't actually use them; when creating precompiled headers
with Visual C++, a dummy .cpp is created at build time.

Also cv::, cv::gpu:: and cv::ocl:: namespace prefixes can be safely omitted
inside CV_ENUM and CV_FLAGS

* added DenseOpticalFlow interface
* moved OpticalFlowDual_TVL1 to src folder

Added new performance tests for calcOpticalFlowPyrLK and buildOpticalFlowPyramid; extracted private header from lkpyramid.cpp