UI improvement for changing attribute color mapping

[chkw]

We are constantly being asked to change attribute colors. We need an easier way to do it.

1) Changing the color mapping for an attribute should be a button available on the attribute from the short list.

2) A colormap should appear where the actual colors can be selected, possibly from a color wheel. It is way too cumbersome the way its implemented now.

[terraswat]

Requested again at presentation 2/28/2018.

[terraswat]

This will go in with the context menu

[terraswat]

implemented