Merge pull request #13120 from dbudniko:dbudniko/gapi_opencl_kernel_example

Merge pull request #13120 from dbudniko:dbudniko/gapi_opencl_kernel_example

* custom OpenCL G-API kernel draft

* clean up and warnings fix

* more warnings

* white space

* new blank line at the EOF removed

* HAVE_OPENCL guard

* remove unnecessary ocl API call

* remove sum test workaround

* check if opencl activated

* fix std::str warning

* CPU fall back for symm7x7

* gpu test kernel draft

* adjust have opencl guard

* more guards

* one more attempt to adjust guards

* empty stub files and kernel source files creation in the test directory

* try to force auto generation

* one more attempt to force build

* remove symm7x7 custom from gapi module

* looks like that this version works properly on Win desktop

* clean up

* more clean up

* address some suggestions from Dmitry's review

* const kernel coefficients

* CV_Error in kernel + try to fix cpu fallback

* CV_Error_ instead CV_Error

* everything in one gapi_gpu_test.cpp

* fix warning

* remove kernel generation, add kernel string

* avoid generated code and ocl internal namespace

* fix misprint

* c_str