[Kst] Image data type

[Rick Chern]

I am currently working on an Image data type.  Description of images:
- images will be specified by three vectors, representing x, y, and z
- it is up to the user to make sure the three vectors selected make 
sense as an image
- images will be plotted on the 2D plot, using colours/shades to represent z
- only 1 image can be plotted per plot
- images will be plotted underneath any curves
- images will appear as new data objects in the data manager

Changes I would make:
- create a KstImage class inheriting from KstDataObject (not KstBaseCurve)
- add Image button to the New section of data manager
- modify plot dialog slightly to be able to select 1 image (or not)
- create an image New/Edit dialog
- make changes to Kst2DPlot to set, remove, and plot the image, if the 
plot has one

I just wanted to check to be aware of any problems before I went to far 
- some of the changes have been made, but not committed yet.

Rick.