Implement double-click column autosize for grids

WxWidgets has this lovely disclaimer:

    // override to implement column auto sizing
    virtual bool UpdateColumnWidthToFit(unsigned int idx, int widthTitle)
    {
        // TODO: currently grid doesn't support computing the column best width
        // from its contents so we just use the best label width as is
        GetOwner()->SetColSize(idx, widthTitle);

        return true;
    }

... so it looks like we'll have to do it ourselves.