Adding multiple columns is cumbersome

Users sometimes want to add multiple columns. The nature of the class tree makes this cumbersome.

```
> Manage column: Add column: scroll down to find protein with gene: want to
> add "length" and "Molecular Weight".
>
> Scroll, find Protein under gene, click to open, scroll, select the
> first. Have to repeat all of those steps again for MW. Would be
> nice if remembers location and maybe highlight already selected
> columns as that gives landmarks as one scrolls rapidly without
> reading...
```

The initial behaviour was to allow multiple selection of columns; maybe this could be re-enabled, possibly as a runtime switch or user opt-in.