Highly speed up code-completion with a very long completion-list. Steps to achieve this:
- Don't use indexOf(..) in the row list when inserting items, instead do a binary search using qUpperBound
- Cache the filter-text
- Don't do a complete re-sort after creating the groups, the order is already consistent after the group-creation.

The functionality to allow reverse sorting was removed on the way, it was too much complication for too low benefit, coming in the way.