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* Remove out of date tests

G-API: Doxygen class reference

* G-API Doxygen documentation: covered cv::GComputation

* G-API Doxygen documentation: added sections on compile arguments

* G-API Doxygen documentation: restructuring & more text

* Added new sections (organized API reference into it);
* Documented GCompiled, compile args, backends, etc.

* G-API Doxygen documentation: documented GKernelPackage and added group for meta