makeArrayGadget improvements

[gergness]

The makeArrayGadget() doesn't do quite what Matt wanted. I also saw Natlia is starting design work on this exact same issue, so I don't know if we should work on the R package as a way to take pressure off, or if it would take away from the web app.

Current workflow:Screen 1:

Screen 2:

After hitting "Done" in top right, code is inserted into the editor pane.

Proposed improvements:

1. Add option to choose between using makeArray and deriveArray (meaning existing variables aren't always forced to move inside array) (toggle on second screen)

2. Add ability to select a single existing array variable (as an alternative to the current behavior where you can select multiple existing 1D variables)

3. Remove "Done" button on the second screen, replace with buttons on bottom to either 1) Copy code to editor, or 2) Run code

[malecki]

1: is it feasible to always derive and not make users have to know about the distinction?
2,3: if possible — any sense of complexity/estimate?