Merge pull request #13851 from TolyaTalamanov:at/new-kernel-package-design

G-API: Kernel package design (#13851)

* Remove cv::unite_policy from API

* Add check that all id in kernel package are unique

* Refactor checker id procedure

* Remove cv::gapi::GLookupOrder from API

* Implement cv::gapi::use_only

* Fix samples

* Fix docs

* Fix comments to review

* Remove unite_policy

* Fix GKernelPackage::backends()

* Fix comments to review

* Fix all_unique

* Fix comments to review

* Fix comments to review

* Remove out of date tests

modules/gapi/samples/api_ref_snippets.cpp

@@ -49,8 +49,7 @@ int main(int argc, char *argv[])
     //! [apply_with_param]
     cv::gapi::GKernelPackage kernels = cv::gapi::combine
         (cv::gapi::core::fluid::kernels(),
-         cv::gapi::imgproc::fluid::kernels(),
-         cv::unite_policy::KEEP);
+         cv::gapi::imgproc::fluid::kernels());
     sobelEdge.apply(input, output, cv::compile_args(kernels));
     //! [apply_with_param]