zjquery: Fix chaining semantics.

We now make it so that $('foo').addClass(whatever) and similar
functions properly return the wrapper object for chaining
purposes.  We may eventually want to change the wrapper object
to automatically dispatch to the first child object, but this
should work for now.

frontend_tests/zjsunit/zjquery.js

@@ -58,11 +58,11 @@ exports.make_zjquery = function () {
             },
             addClass: function (class_name) {
                 classes.set(class_name, true);
-                return self;
+                return self.wrapper;
             },
             removeClass: function (class_name) {
                 classes.del(class_name);
-                return self;
+                return self.wrapper;
             },
             hasClass: function (class_name) {
                 return classes.has(class_name);
@@ -108,7 +108,7 @@ exports.make_zjquery = function () {
                 return self;
             },
             on: function () {
-                return self;
+                return self.wrapper;
             },
             remove: function () {
                 if (my_parent) {
@@ -127,6 +127,7 @@ exports.make_zjquery = function () {
                 result[attr] = elem[attr];
             }
         }
+        elem.wrapper = result;
 
         return result;
     }