Click terms to fill MatrixEditor entries

[ghost]

It's a real pain to enter things into the popover EXTMatrixEditors once they have sufficiently many rows. Since many of these are tied to visible terms in the chart, it would be nice to be able to go through the following steps:

1. Open such a popover.
2. Highlight one of the columns (or at least one of the cells) in one of the matrix editors.
3. Click a term in the chart.
4. (If the click makes sense in context, then) Populate the selected column with a representative of the clicked term.

This might take some work, tracking the UI click and figuring out what sits under it. Still would be nice.