Merge remote-tracking branch 'upstream/3.4' into merge-3.4

diff --cc CMakeLists.txt
Simple merge

diff --cc cmake/OpenCVModule.cmake
Simple merge

diff --cc modules/core/include/opencv2/core/types_c.h
Simple merge

diff --cc modules/core/src/ocl.cpp
Simple merge

diff --cc modules/core/src/utils/filesystem.cpp
Simple merge

diff --cc modules/dnn/src/layers/elementwise_layers.cpp
Simple merge

diff --cc modules/dnn/src/onnx/onnx_importer.cpp
Simple merge

diff --cc modules/dnn/test/test_backends.cpp
Simple merge

diff --cc modules/highgui/src/precomp.hpp
Simple merge

diff --cc modules/highgui/src/window.cpp
Simple merge

diff --cc modules/highgui/src/window_w32.cpp
Simple merge

diff --cc modules/imgproc/src/contours.cpp
Simple merge