Canvas: Fix placing existing canvas on window.

- Canvas window placements now use 'id' instead of 'index'...
 - Provide Dialog class for backwards compatibilty (but print a
   warning that it will be removed)

Nasal/canvas/gui.nas

@@ -71,7 +71,7 @@ var Window = {
     if( !isa(canvas_, canvas.Canvas) )
       return debug.warn("Not a canvas.Canvas");
 
-    canvas_.addPlacement({type: "window", index: me._node.getIndex()});
+    canvas_.addPlacement({type: "window", "id": me.get("id")});
     me['_canvas'] = canvas_;
   },
   # Get the displayed canvas
@@ -214,3 +214,12 @@ var Window = {
     });
   }
 };
+
+# Provide old 'Dialog' for backwards compatiblity (should be removed for 3.0)
+var Dialog = {
+  new: func(size, type = nil, id = nil)
+  {
+    debug.warn("'canvas.Dialog' is deprectated! (use canvas.Window instead)");
+    return Window.new(size, type, id);
+  }
+};