Group panel, remove configured group error.

```
TypeError: col is undefined
http://debian/fsp/dist/js/vendor/KoGrid/koGrid-2.0.6.debug.js
Line 1516
```

At:

```
col.isGroupedBy(false);
```

Actually: `Compiled At: 01/11/2013 15:58:36`, just pasted in that file.

Reproduce:
- `showGroupPanel: true`.
- `groups: ['parent_id']`. (without hard-coded groups it works as intended)
- Manually add another group field. (actually not required to error)
- Attempt to remove first group field with header close button.

Also triggered on group buttons in filter drop-down when a pre-set group is clicked (off). Think the `isGroupedBy` isn't being set by pre-set config.

Start at dist/js/vendor/KoGrid/koGrid-2.0.6.debug.js around line 1516, where col.isGroupedBy(false) is called. Reproduce with showGroupPanel enabled and groups set to ['parent_id'], then remove the first group or toggle it from the filter dropdown. Done means these actions no longer produce the TypeError.

Written by the indexing model from the issue text.