Add X/Y axis dropzones to scatterplot tab

[bherr2]

No description provided.

[bherr2]

Things look to be mostly working now on develop, even on firefox.

Some niggling issues:

• Sometimes on Firefox, when you start dragging, all the droppable regions stop being highlighted but are still droppable if you drag over it. Drag/drop highlighting on Firefox stop randomly #256
• I had to put a hack in to the visualization-view/main/main.component.ts to fully unload the scatterplot on graphic var change because its not properly handling those changes. GV change not being handled properly #257
• Scatterplot is not properly handling categorical variables (see fav activity or age group) Scatterplot X/Y axis sorting of categorical variables #258

[bherr2]

I think ultimately we need to replace most of the drag-drop plumbing with Angular CDK Drag & Drop code

https://material.angular.io/cdk/drag-drop/overview

[bherr2]

Scatterplot categorical variables should be supported when we publish the new dvl-fw which properly sets a dataType. See cns-iu/make-a-vis@fe8e324