Improve coherence in Aggregate: Add a New Grouped Attribute

Hi, 

Today, to create a new grouped attribute, you need to open an attribute menu in order to do it, while to add a new attribute (not grouped), you have a column at the end of the aggregate:

The "Add new attribute" in a context menu is not intuitive, and is not coherent with the most used "New Attribute" column.

I would like to see the same behavior: A column at the end of the Grouped section with a New Group Attribute. It would be more intuitive, and would be the same behaviour we alread have in the filters, where an Add Group Filter appears when you have a grouped attribute.