Bad refs to rowColumnIndex and treeColumnIndex

[These lines](https://github.com/fin-hypergrid/core/blob/develop/src/renderer/index.js#L661-L667) , the context for`rowColumnIndex` and `treeColumnIndex` should be `this.grid.behavior` (not `this.grid`).

This resulted in `getColumnWidth()` always returning the default column width instead of the actual column width (not obvious when the actual column with _was_ the default).

This is a problem in develop, introduced in #598: _fix keyboard navigation_; it is not a problem in master.

Open src/renderer/index.js at lines 661-667 and inspect the references used by getColumnWidth(). Verify the lookup uses the behavior context described in the issue, then confirm that a non-default actual column width is returned; the fix should affect develop only, since master is not reported as affected.

Written by the indexing model from the issue text.