[GAPI] RMat integration into the framework

* RMat integration

* Added initialization of input mat in GArray initialization tests

* Fixed klocwork warnings in RMat tests, changed argument order in EXPECT_EQ

* Modified for Intel Realsense webcam

* Fix whitespace

* Update page title

* Uppercase

[G-API] Export a part of serialization interface

* Initial stub

* Add test on serialization of a custom type

* Namespaces rework

* Fix isSupported in test struct

* Fix clang build and rework namespaces

* Remove redundant header

[G-API]: countNonZero() Standard Kernel Implementation

* Add countNonZero() standard kernel
 - API and documentation provided
 - OCV backend supported
 - accuracy and performance tests provided
 - some refactoring of related documentation done

* Fix GOpaque functionality for OCL Backend
 - test for OCL Opaque usage providied

* countNonZero for GPU
 - OCL Backend implementation for countNonZero() added
 - tests provided

* Addressing comments

* G-API: Introduce cv::MediaFrame, a host type for cv::GFrame

* G-API: RMat -- address review comments

[G-API] Add support for more types serialization

* Support more types

* Add std::string support

* Fix GOpaque and gin interaction

* Fix tests on kind

* Make map serialization support templates and add tests on kind