Set status to beta
OpenNI2: prevent failure of setVideoMode() call with invalid parameters

Not all parameters are specified for openni::VideoMode, so
"selected" mode can be unsupported for device.
Replace default VideoMode constructor to result of getVideoMode() call.
openni_capture sample: enable OpenNI2 if available

Sample works with OpenNI2 with minimal code changes (CAP_OPENNI -> CAP_OPENNI2)
Merge pull request #3404 from alalek:openni2_patch
Merge pull request #3405 from D-Alex:master
Merge tag '3.0.0-beta'

OpenCV 3.0.0 Beta
Mod: Open header files with `io.open` for Python 2/3 compatibility.
fixed doc
Merge pull request #3408 from ilya-lavrenov:doc_fix
Fix Cuda -Wl linker options

This line causes the following error when building custom projects that use OpenCV beta 3:
ld: warning: directory not found for option '-L/<project_root>/-Wl,/usr/local/cuda'
Merge pull request #3410 from mgharbi:master
Fix canny segment fault (Bug #3978)

Avoid stack overflow based on std::vector cause
std::vector::resize() segmentation fault.
Merge pull request #3407 from wangyan42164:canny_bug_fix
Doxygen documentation for core module
Merge pull request #3406 from shiquanwang:Fix-bug-#4003_build-python-binding-under-python3
Improve the performance of fast_nlmeans_denoising_opencl.

1. Remove unnecessary barriers.
2. Adjust CTA_SIZE based on the following cases for Intel platform:
   a) OCL_Photo_DenoisingGrayscale.DenoisingGrayscale
   b) OCL_Photo_DenoisingColored.DenoisingColored
fix gpu performance sample compilation
Merge pull request #3413 from wangyan42164:denoising_opencl_improvement
Merge pull request #3415 from jet47:fix-gpu-perf-sample
Merge pull request #3399 from mshabunin:doxygen-core