backport of commit: 59b0314a

OpenCv.js docker container build instructions (#15221)

* emscripten docker container

* docker

* typo

* typo

* whitespace issue

Merge two Haar Cascade tutorials (#14674)

* move haar cascade introduction, add code explanation, mark content as moved

* switch to ref for include to provide correct breadcrumb navigation

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* Remove out of date tests

Add SOLVEPNP_IPPE for planar pose estimation. Add solvePnPGeneric function that returns all the pose solutions and the reprojection errors.

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

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

Add solvePnPRefineLM to refine a pose according to a Levenberg-Marquardt iterative minimization process. Add solvePnPRefineVVS to refine a pose using a virtual visual servoing scheme.

Fixes a broken link for histograms of Oriented Gradients.

fix just a typo of the word 'word' (#14297)

- replace tutorial links via docs.opencv.org
- remove link on OpenCV 2.4
- avoid links on outdated packages

* improve thresholding tutorial, fix grammar issues and incorrections

* keep full list of simple thresholding types

* Fixed #14148 Broken Documentation Link

* fixup: use 3.4 branch

* Created python version of the code for the anisotropic image segmentation tutorial. Created python/cpp toggles for the markdown file.

* fix doxygen warnings

Must have that flag. Otherwise following "doxygen overview" tutorial won't work.

Changes doc, images and sample code itself

backport 997b7b18

* Add note about building for pkg-config

* minor fixes

* fixed tutorial code of py_matcher

* fixed imread mode

* Split videoio dependencies search into pieces, removed obsolete backends

OpenCV 3.4.5