Ensure dictionary type instance persistence

Adds an assignment function for dict types that preserves the dict
instances on the three objects.

Author: vidartf

js/scripts/prop-types.js

@@ -364,6 +364,9 @@ _.extend(DictType.prototype, BaseType.prototype, {
         var nullableStr = this.nullable ? 'True' : 'False';
         return `Dict(default_value=${this.getPythonDefaultValue()}, allow_none=${nullableStr}).tag(sync=True)`;
     },
+    getPropertyAssignmentFn: function() {
+        return 'assignDict';
+    },
 });
 
 function FunctionType(fn) {

js/src/_base/Three.js

@@ -644,6 +644,18 @@ var ThreeModel = widgets.WidgetModel.extend({
         return threeType.ipymodel;
     },
 
+    // Dict
+    assignDict: function(obj, key, value) {
+        if (obj[key] === value) {
+            // If instance equality, do nothing.
+            return;
+        }
+        // Clear the dict
+        Object.keys(obj[key]).forEach(k => { delete obj[key][k]; });
+        // Put in the new values
+        Object.assign(obj[key], value);
+    },
+
     // ThreeTypeArray
     convertThreeTypeArrayModelToThree: function(modelArr, propName) {
         return modelArr.map(function(model) {