G-API aux kernels (#14599)

* Removed gcpuimgproc.hpp and gcpucore.hpp

* Changed GModel::orderedInputs and orderedOutputs to get ConstGraph&

* Added auxiliaryKernels() method of GBackend::Priv

G-API: own::Mat as output computation doesn't work (#14151)

* Fix bug with output own::Mat

* Fix comments to review

* Fix preprocess condition

* Fix comments to review

G-API: GComputation doesn't work with output vector<cv::Mat> (#14150)

* Fix bug with gcomputation output vector

* Add test fixture

* Fix comments to review

* Fix alignment

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* Remove out of date tests

* Fix typo: 'divisble' -> 'divisible'

* Fix typo: 'One of arguments' -> 'One of the arguments'

Extend optical flow tutorial (#14314)

* extend python optical flow tutorial with cpp example code and add it to general tutorial directory

* remove unused parameters, fix comparison between signed and unsigned int

* fix hsv range problem

* switch to samples::findFile for sample file location

* switch to command line parameter for path

* remove old tutorial as in 14393

* minor fixes

'cv::String' is 'std::string' in OpenCV 4.x

Extend meanshift tutorial (#14393)

* copy original tutorial and python code

* add cpp code, fix python code

* add camshift cpp code, fix bug in meanshift code

* add description to ToC page

* fix shadowing previous local declaration

* fix grammar: with -> within

* docs: remove content of old py_meanshift tutorial, add link

* docs: replace meanshift tutorial subpage in Python tutorials

* switch to ref to fix wrong breadcrumb navigation

* switch to cmdline for path as in #14314

* Apply suggestions from code review

* order programming languages alphabetically

- moved code into separate file due issue with handling of CMake policies