Implements #6544 (eliminates 10/42 of the constantly-displayed draw calls).
Fixes#6717. Improves startup CPU time by 5% (250ms, loading Orders on my dev box).
# Important Notes
- Edges: New implementation uses only Rectangle under most conditions.
- Node and action area: Replace some shapes with Rectangle.
- List view: Replace some shapes with Rectangle.
- Display object hierarchy: The lowest-level shape instance types no longer have their own display objects.
- Includes initial support for using `Rectangle` to display triangles.
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