BUGFIX:  This change fixes #2475, where contour labels added manually

produce ugly spikes in the contours.  The problem was that screen
coordinates were being passed as data coordinates when the contour was
split to add room for the label.

Incorporate @bwkeller fix and @miili's #2806 change

See long comment on #2818 for why this is the correct fix.
On second thought, the docstring could be made clearer:
"Add a label near the point (x, y). If *transform* is None (default), (x, y) is in data coordinates; if *transform is False, (x, y) is in display coordinates; otherwise, the specified transform will be used to translate (x, y) into display coordinates."

Maybe something like that would be clearer.