* fix test failure of cudaimgproc convert color bgr2lab series

Objdetect: QRCode detect + decode

* samples: add winpack "drap & drop" build script

* samples: add search for MSVS 2017 Enterprise

* Add documentation for cvtColorTwoPlane

* Change brief and add links

* Add Shufflenet support in ONNX

* Add test for transpose layer

* cmake: don't ignore Python from PATH environment variable

- this breaks selection between 32/64-bit Python
- this breaks Anaconda/Conda environments
- it is not the CMake default behavior, expected by many projects

* cmake: add Python version check, fallback path on CMake 3.12+

* cmake: drop Python 2.6, allow version selection for Python 3.x

especially in CPU only processing

_tzcnt_u32() is not exported by clang-cl intrin.h so check for
clang-cl and enable an alterate for _tzcnt_u32()

Some discussions:
http://lists.llvm.org/pipermail/cfe-dev/2016-October/051329.html
https://bugs.llvm.org/show_bug.cgi?id=30506

TEST=Build with clang-cl

to avoid build break without `__OPENCV_BUILD`