Rect: split into Heatmap and Histogram

[TCL735]

Currently the "rect" layer type can be both a Heatmap or a Histogram. While that works for now, it will be harder to maintain when Heatmap and Histogram need their own improvements and updates.

Split the Rect component into two separate components to allow Heatmap and Histogram to diverge independently.

[kristinarobinson]

Low priority - until approval for viz types goes forward.