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

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

diff --cc modules/dnn/src/tensorflow/tf_importer.cpp
Simple merge

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

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

diff --cc modules/imgproc/include/opencv2/imgproc.hpp
Simple merge

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