Fix a code indent bug:   cv2.imshow("detected lines", cdst)

Moved second `imshow()` inside the if-statement to prevent error when no lines have been found and the function is called with an empty `cdst`.

In the C++ equivalent of this example a check is made whether the vector (here in Python we have a list) actually has any lines in it that is whether the Hough lines function has managed to find any in the given image. This check is missing for the Python example and if no lines are found the application breaks.

Prevent grabcut, houghcircles and houghlines python2 demos from running automatically while browsing demos in the demo list viewer

fix whitespace

houghlines.py for HoughLinesP() and HoughLines()
houghcircles.py for HoughCircles()