Update widget serializer/unserializer to allow custom serialization on a trait-by-trait basis.

Serializers/unserializers are given as the trait metadata keys `to_json` and `from_json`.

IPython/html/widgets/widget.py

@@ -195,8 +195,16 b' class Widget(LoggingConfigurable):'
             A single property's name to get.
         """
         keys = self.keys if key is None else [key]
-        return {k: self._pack_widgets(getattr(self, k)) for k in keys} 
-
+        state = {}
+        for k in keys:
+            f = self.trait_metadata(k, 'to_json')
+            value = getattr(self, k)       
+            if f is not None:
+                state[k] = f(value)
+            else:
+                state[k] = self._serialize_trait(value)
+        return state
+    
     def send(self, content):
         """Sends a custom msg to the widget model in the front-end.
 
@@ -280,7 +288,11 b' class Widget(LoggingConfigurable):'
         """Called when a state is received from the front-end."""
         for name in self.keys:
             if name in sync_data:
-                value = self._unpack_widgets(sync_data[name])
+                f = self.trait_metadata(name, 'from_json')
+                if f is not None:
+                    value = f(sync_data[name])
+                else:
+                    value = self._unserialize_trait(sync_data[name])
                 with self._lock_property(name, value):
                     setattr(self, name, value)
 
@@ -299,31 +311,34 b' class Widget(LoggingConfigurable):'
         """Called when a view has been displayed for this widget instance"""
         self._display_callbacks(self, **kwargs)
 
-    def _pack_widgets(self, x):
-        """Recursively converts all widget instances to model id strings.
+    def _serialize_trait(self, x):
+        """Serialize a trait value to json
 
-        Children widgets will be stored and transmitted to the front-end by 
-        their model ids.  Return value must be JSON-able."""
+        Traverse lists/tuples and dicts and serialize their values as well.
+        Replace any widgets with their model_id
+        """
         if isinstance(x, dict):
-            return {k: self._pack_widgets(v) for k, v in x.items()}
+            return {k: self._serialize_trait(v) for k, v in x.items()}
         elif isinstance(x, (list, tuple)):
-            return [self._pack_widgets(v) for v in x]
+            return [self._serialize_trait(v) for v in x]
         elif isinstance(x, Widget):
             return x.model_id
         else:
             return x # Value must be JSON-able
 
-    def _unpack_widgets(self, x):
-        """Recursively converts all model id strings to widget instances.
+    def _unserialize_trait(self, x):
+        """Convert json values to objects
 
-        Children widgets will be stored and transmitted to the front-end by 
-        their model ids."""
+        We explicitly support converting valid string widget UUIDs to Widget references.
+        """
         if isinstance(x, dict):
-            return {k: self._unpack_widgets(v) for k, v in x.items()}
+            return {k: self._unserialize_trait(v) for k, v in x.items()}
         elif isinstance(x, (list, tuple)):
-            return [self._unpack_widgets(v) for v in x]
-        elif isinstance(x, string_types):
-            return x if x not in Widget.widgets else Widget.widgets[x]
+            return [self._unserialize_trait(v) for v in x]
+        elif isinstance(x, string_types) and x in Widget.widgets:
+            # we want to support having child widgets at any level in a hierarchy
+            # trusting that a widget UUID will not appear out in the wild
+            return Widget.widgets[x]
         else:
             return x