Feature request: remove existing root annotation automatically when enter a new annotation to the same aspect.

[hdrabkin]

This is a request for a trigger we had in the MGI GO annotation module. If there were existing root annotations for a gene, the upon entering a new annotation to the same aspect, the root annotation was automatically removed. At this time, either in form editor or graph editor, this does not happen. When there are many annotations in the model, it is often hard to see the root annotations to remove, or, one forgets to remove them.