Merge remote-tracking branch 'origin/2.4'

Merge remote-tracking branch 'origin/2.4'

Conflicts:
	doc/tutorials/features2d/feature_detection/feature_detection.rst
	modules/bioinspired/doc/retina/index.rst
	modules/core/include/opencv2/core/core.hpp
	modules/core/include/opencv2/core/mat.hpp
	modules/core/include/opencv2/core/operations.hpp
	modules/core/src/stat.cpp
	modules/features2d/include/opencv2/features2d/features2d.hpp
	modules/imgproc/src/filter.cpp
	modules/legacy/src/dpstereo.cpp
	modules/nonfree/src/surf.ocl.cpp
	modules/ocl/doc/image_processing.rst
	modules/ocl/doc/object_detection.rst
	modules/ocl/include/opencv2/ocl/ocl.hpp
	modules/ocl/include/opencv2/ocl/private/util.hpp
	modules/ocl/src/arithm.cpp
	modules/ocl/src/canny.cpp
	modules/ocl/src/filtering.cpp
	modules/ocl/src/imgproc.cpp
	modules/ocl/src/initialization.cpp
	modules/ocl/src/matrix_operations.cpp
	modules/ocl/src/pyrdown.cpp
	modules/ocl/src/pyrup.cpp
	modules/ocl/src/split_merge.cpp
	modules/ocl/test/test_objdetect.cpp
	modules/ocl/test/utility.hpp