[Kst] [Bug 115970] Datamode doesn't work on images.

[Barth Netterfield]

Non data mode:
	whether or not curves exist:
		Not over image: 
			x,y of the cursor
		Over image: 
			x,y,z of image under the cursor.

Data mode:
	If curves exist: 
		If over image:
			x,y of nearby curve, z of image under that point.
		if not over image:
			x,y of nearby curve
	If curves do not exist
		Same as non-data mode