TreeDataGrid - Entirety of group row to trigger toggling

## Use case
When using the `TreeDataGrid` (latest version), I have to click on the first cell of the group row in order to toggle it. Clicking on the rest of the row (on any of the cells ) doesn't do anything.

This can be seen here: https://adazzle.github.io/react-data-grid/#/RowGrouping.

If I click on the cell of Zimbabwe, the toggle works as expected. However, if I click on any other cell, the toggle isn't triggered.

## Proposed solution

I would like for the whole group row's cells to perhaps have the click handler to trigger the toggling.

## Use case

This would be useful for big grids (on smaller screens), where horizontal scrolling takes place, and the initial group column goes out of view.