Merge pull request #14599 from rgarnov:gapi_aux_kernels

G-API aux kernels (#14599)

* Removed gcpuimgproc.hpp and gcpucore.hpp

* Changed GModel::orderedInputs and orderedOutputs to get ConstGraph&

* Added auxiliaryKernels() method of GBackend::Priv

modules/gapi/src/api/gbackend.cpp

@@ -45,6 +45,11 @@ void cv::gapi::GBackend::Priv::addBackendPasses(ade::ExecutionEngineSetupContext
     // add custom (backend-specific) graph transformations
 }
 
+cv::gapi::GKernelPackage cv::gapi::GBackend::Priv::auxiliaryKernels() const
+{
+    return {};
+}
+
 // GBackend public implementation //////////////////////////////////////////////
 cv::gapi::GBackend::GBackend()
 {