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.