massive Shape rework: keep both tvPoints/Props and ourPoints/Props; delegate model updates to subclasses; DCA/VLine working but Ladder/HLine not done.

src/common.js

@@ -3,7 +3,7 @@ export function mixin(child, ...parents) {
     // assigned by parents order, highest priority first
     for( const parent of parents ) {
         for ( const key in parent) {
-            if (parent.hasOwnProperty(key) && !child.hasOwnProperty(key)) {
+            if (parent.hasOwnProperty(key) && !child.hasOwnProperty(key) && parent[key] !== undefined) {
                 child[key] = parent[key];
             }
         }
@@ -11,7 +11,6 @@ export function mixin(child, ...parents) {
     return child;
 }
 
-
 export function prototype(parent, child) {
     const result = Object.create(parent);
     Object.assign(result, child)