AttributeUi: would be nice if clicking a terminal item would allow you to add it to the query

[rpbouman]

Currently we can add attributes to the query by clicking the respective axis button, or by dragging and dropping.
Usability could be further improved by also letting a single click on terminal items add the item to the query.

Items that could be added to multiple axes should pop up a menu with the available choices; items that can only be added to one axis (aggregates) could simply carry out the action.