Add Sketcher commands, toolbar items, menu items, and icons for ellipse, parabola, and hyperbola.

Ellipse icon is from abdullah's ellipse branch; the other two are my own quick hacks.

None of the commands do anything yet.
Initial work on implementing the DrawSketchHandlerEllipse class. It works well, but saving the ellipse isn't implemented yet.
Robust, documented, and working version of my implementation of the DrawSketchHandlerEllipse class.

It fixes some drawing issues and edge-case bugs in abdullah's version.  It also incudes some othet bits to make the saving work.

I still need to go over the suggested constraint parts.