Merge pull request #16889 from mehlukas:3.4-consolidateImshow

* consolidate tutorials on image read/display/write

* fix unsused variables

* apply requested changes

* apply requested changes

* fix mistake

samples/python/tutorial_code/introduction/display_image/display_image.py

@@ -0,0 +1,19 @@
+## [imports]
+import cv2 as cv
+import sys
+## [imports]
+## [imread]
+img = cv.imread(cv.samples.findFile("starry_night.jpg"))
+## [imread]
+## [empty]
+if img is None:
+    sys.exit("Could not read the image.")
+## [empty]
+## [imshow]
+cv.imshow("Display window", img)
+k = cv.waitKey(0)
+## [imshow]
+## [imsave]
+if k == ord("s"):
+    cv.imwrite("starry_night.png", img)
+## [imsave]