Changed heatmap rendering.

It is now rendered using a sequence of drawing primitives instead of a
large pixmap.

Also implemented adaptive contingency resampling.
Speedup heatmap painting.

(bound the drawing to the exposed area only)
Added heatmap widget icon.
Removed unused code.
Painting code cleanup.
Moved the sharpen_region function out the OWHeatmap class.
Implemented recursive heatmap sharpening.
Re-enabled ordering by chi square.
Merge pull request #103 from ales-erjavec/heatmap

Heatmap