gapi: Full calcOpticalFlowPyrLK implementation (2 overloads) and tests

- opencv_gapi module is linked with opencv_video module (optional dependency)
     - kernels added to a new cv::gapi::video namespace and a brand new files created to provide gapi_video environment
     - there are 2 different kernels as G-API should provide GMat AND GArray<GMat> implementation: cv::calcOptFlowPyrLK doesn't calculate pyramids if vector<Mat> is given so just the cast GMat -> GArray<GMat> wouldn't represent all the cv:: functionality
     - tests to check both kernels (based on cv::video tests for cv::calcOpticalFlowPyrLK())
     - tests for internal purposes added
     - vectors<T> comparison in tests implemented
     - new (and old too) common test structures refactored to avoid code copypasting
     - "modules/gapi/test/common/gapi_video_tests_common.hpp" created to share some code snippets between perf and acc tests and avoid code copypasting

modules/gapi/perf/cpu/gapi_imgproc_perf_tests_cpu.cpp

@@ -220,4 +220,4 @@ INSTANTIATE_TEST_CASE_P(RGB2YUV422PerfTestCPU, RGB2YUV422PerfTest,
         Combine(Values(ToleranceColor(1e-3).to_compare_f()),
             Values(szVGA, sz720p, sz1080p),
             Values(cv::compile_args(IMGPROC_CPU))));
-}
+} // opencv_test