Various minor tweaks

Added comments, renamed files, removed cruft.

Stop showing the visualization tag name in the code list.  It's
often redundant with the code label, and it's distracting.  (We may
want to make this an option so you can Ctrl+F to find a tag.)

SourceGen/MainController.cs

@@ -3623,7 +3623,6 @@ namespace SourceGen {
                     }
                     break;
                 case LineListGen.Line.Type.VisualizationSet:
-                    // TODO(xyzzy)
                     lineTypeStr = "visualization set";
                     break;
                 default: