Merge pull request #13240 from dbudniko:dbudniko/gapi_gpu_to_ocl_renaming

G-API rename GPU backend to OCL backend (#13240)

* renaming draft

* inline namespace instead non-safe define

* more back compatibility

* Updates after review from Dmitry

modules/gapi/test/gpu/gapi_imgproc_tests_gpu.cpp

@@ -8,7 +8,6 @@
 #include "../test_precomp.hpp"
 
 #include "../common/gapi_imgproc_tests.hpp"
-#include "opencv2/gapi/gpu/imgproc.hpp"
 
 #define IMGPROC_GPU cv::gapi::imgproc::gpu::kernels()