[Cin] Chromakey improvements checked in to GIT

Chromakey plugin has had its menu improved by anonymous contribution and it has been checked into GIT. (Andrea, can you test it too? and check the manual to make sure still matches and make new image?) Improvements include: - added Default button - default values for Threshold and Slope changed from 60 to 10 and from 100 to 0 as much more practical - position of Threshold and Slope switched because Threshold is more relevant (Previous 2 items were suggested by Andrea) - textbox with arrows added for much easier access to current setting - individual "reset" icons added - location of "Use color picker" put into better position - other minor stuff