[Feature Request] Navigator - allow grouping by relation and target element type

Pain: When the active element has many relations (think 10+) with different types of other elements the navigator lists becomes very long.

Request:
Enable the grouping feature in the navigator tree. Options:

- no grouping (i.e. as is)
- group by relation type
- group by other element's type

Start at the navigator tree and trace how relations of the active element are currently listed. Implement the three requested modes—no grouping, grouping by relation type, and grouping by the other element's type—and verify that each mode produces the corresponding navigator structure.

Written by the indexing model from the issue text.