Progress toward wireframe animations

Handle the remaining visualization editor UI controls, except for
the "test" button.  Save/restore wireframe animations in the
project file.  Changed the preview from a 1-pixel-wide line drawn
by a path half the window size to a 2-pixel-wide line drawn by a
path the exact window size.

SourceGen/Visualization.cs

@@ -192,7 +192,7 @@ namespace SourceGen {
         /// </summary>
         /// <param name="visWire">Visualization object.</param>
         /// <param name="parms">Visualization parameters.</param>
-        public void SetThumbnail(IVisualizationWireframe visWire,
+        public virtual void SetThumbnail(IVisualizationWireframe visWire,
                 ReadOnlyDictionary<string, object> parms) {
             Debug.Assert(visWire != null);
             Debug.Assert(parms != null);