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