Handle special keys in waitKey(), and implement waitKeyEx()

[sfe-SparkFro]

micropython-opencv/src/highgui.cpp

Lines 91 to 93 in a2bf9f2

	// TODO: Some key presses return multiple characters (eg. up arrow key
	// returns 3 characters: "\x1b[A"). Need to handle this case properly.
	// Should also look into implementing waitKeyEx() for these extra cases