move "hidden" constructor from api.nas to api/canvas.nas; add destructor support

Nasal/canvas/api.nas

@@ -39,14 +39,11 @@ Group._element_factories = {
 io.include(include_path~"canvas.nas");
 
 # @param g Canvas ghost
-var wrapCanvas = func(g)
-{
-  if( g != nil and g._impl == nil )
-    g._impl = {
-      parents: [PropertyElement, Canvas],
-      _node: props.wrapNode(g._node_ghost)
-    };
-  return g;
+var wrapCanvas = func(g) {
+    if (g != nil and g._impl == nil) {
+        g._impl = Canvas._new(g);
+    }
+    return g;
 }
 
 # Create a new canvas. Pass parameters as hash, eg:

Nasal/canvas/api/canvas.nas

@@ -5,6 +5,15 @@
 # not to be confused with the namespace canvas (case sensitive!)
 #
 var Canvas = {
+    # private helper for wrapCanvas;
+    _new: func(canvas_ghost) {
+        var obj = {
+            parents: [Canvas, PropertyElement],
+            _node: props.wrapNode(canvas_ghost._node_ghost),
+        };
+        return obj;
+    },
+
     # Place this canvas somewhere onto the object. Pass criterions for placement
     # as a hash, eg:
     #