Merge pull request #15236 from dbudniko:dbudniko/g_api_copy_kernels

diff --cc modules/gapi/include/opencv2/gapi/core.hpp
Simple merge

diff --cc modules/gapi/src/backends/cpu/gcpucore.cpp
Simple merge

diff --cc modules/gapi/src/backends/fluid/gfluidcore.cpp
Simple merge

diff --cc modules/gapi/src/backends/ocl/goclcore.cpp
Simple merge

diff --cc modules/gapi/test/common/gapi_core_tests_inl.hpp
Simple merge