TODO:
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- add side toolbar
- add side feature tree
- add scrolled window around GLWidget
- put GLWidget in a container so it can be hidden

Test Constraints
================
- various n-gons in a circle
- tangent arcs
- concentric circles & arcs

Test Parts
==========
- chain link
- wikipedia's CSG example
- triangle/square/circle part
- box with square holes, round inside/outside edges, corners