G-API: Using functors as kernel implementation
* Implement ability to create kernel impls from functors
* Clean up
* Replace make_ocv_functor to ocv_kernel
* Clean up
* Replace GCPUFunctor -> GOCVFunctor
* Move GOCVFunctor to cv::gapi::cpu namespace
* Implement override for rvalue and lvalue cases
* Fix comments to review
* Remove GAPI_EXPORT for template functions
* Fix indentation