Heatmaps with multiple fields display one per field

[birm]

Heatmaps have one grid, but can have multiple fields on that grid. Now, picking one for a gradient requires a user to open the editor, but instead we can auto-populate the heatmap list with each of the fields separately.

e.g. : a heatmap named "myHeatmap" has "tumor" and "lymph" fields on the same grid. Instead of just displaying "myHeatmap", also display "myHeatmap -> tumor" and "myHeatmap -> lymph" (exact naming structure unimportant)

May have implications to saved heatmap formatting, but ignore for now; just use default colors.

[sohamsshah]

Interesting improvement. Would like to work on this issue. Any specific things to take care while executing this in terms of functionality or code standards?

[birm]

I've removed one subtask which was better stated in #347, and updated this description