[G-API] Allow building graphs with a dynamic number of inputs and outputs
* Add dinamic graph feature and tests
* Remove unnecessary file
* Review response
* Add implementation of operator += for GRunArgs
And test for that case
* Tests refactoring
* Add doxygen
Review response
* Fix docs
* A small documentation fix
* Review response
* Add tests for more entities
* Add typed tests
* Another typed tests
* Doc fix
* Documentation fix
* Build fix
* Commit for rebuild
* The last one
