George Harrison played with many associates of both Dylan and the Stones.    Bobby Keys was one of the most renowned sax men of rock and roll, Nicky Hopkins contributed to the buzz of "Revolution", and Jesse "Ed" Davis had his fingerprints all over early seventies solo Beatles.   This week, we consider some recordings, most made at Apple Studios, most featuring George Harrison on electric (usually slide) guitar.      Who knew George could compose Mexicali blues (as some call "The Holdup")