[GSoC 2019] Improve the performance of JavaScript version of OpenCV (OpenCV.js)

* [GSoC 2019]

Improve the performance of JavaScript version of OpenCV (OpenCV.js):
1. Create the base of OpenCV.js performance test:
     This perf test is based on benchmark.js(https://benchmarkjs.com). And first add `cvtColor`, `Resize`, `Threshold` into it.
2. Optimize the OpenCV.js performance by WASM threads:
     This optimization is based on Web Worker API and SharedArrayBuffer, so it can be only used in browser.
3. Optimize the OpenCV.js performance by WASM SIMD:
     Add WASM SIMD backend for OpenCV Universal Intrinsics. It's experimental as WASM SIMD is still in development.

* [GSoC2019] 

1. use short license header
2. fix documentation node issue
3. remove the unused `hasSIMD128()` api

* [GSoC2019]

1. fix emscripten define
2. use fallback function for f16

* [GSoC2019]

Fix rebase issue

* remove old BUILD_opencv_java option

* replace old build_java option with new BUILD_JAVA option

* removed private flann documentation
* common tutorial images moved to doc/images
* grouping issues

* fix broken code link

resolves  #15344

* Update stitcher.markdown

* update 3.4 link

Found via `codespell -q 3 -S ./3rdparty,./modules -L amin,ang,atleast,dof,endwhile,hist,uint`

backporting of commit: 32aba5e6

backport of commit: 59b0314a

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

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.

backport 997b7b18

* fixed tutorial code of py_matcher

* fixed imread mode

OpenCV 3.4.5