* vget_lane_** accepts constant value, not variable

modules/objectdetect/src/detection_based_tracker.cpp: made unique_lock<mutex> local to each function
samples/cpp/dbt_face_detection.cpp: fixed warnings on loop in Visual Studio

  * use hasSIMD128 rather than calling checkHardwareSupport
  * add SIMD check in spartialgradient.cpp
  * add SIMD check in stereosgbm.cpp
  * add SIMD check in canny.cpp

  * add 8 elements version of reduce operation
  * add tests for new universal intrinsic

checks whether the window exists and is visible. On QT closing a window
merley hides it, so the common hack for checking whether a window exists

exists = cv2.getWindowProperty(.., 0) >= 0

does not work.