Extrude curves

(as requested in #general by Keavon)

- For noncurving polygons, just do extrude (but allow multiselect, so you can extrude multiple polygon segments at a time) (see image below)
- For bezier segments, break the bezier into curve elements, turn these into subdivision lines (2d analog of subdivision surfaces) , and extrude those. You end up with cusps, but that's inevitable.


No file or test is named. Start by locating the existing extrusion behavior for polygon segments and the handling of Bézier segments, then trace how selection and subdivision lines are represented. Done means noncurving polygons support multiselect extrusion and Bézier segments are converted into subdivision lines before extrusion, with the expected cusp behavior.

Written by the indexing model from the issue text.