Support button title change

Change-Id: Ie0f225071edeb2547f841632951715a104477ff7
diff --git a/dev/js/spec/buttongroupSpec.js b/dev/js/spec/buttongroupSpec.js
index 9f20042..189fb24 100644
--- a/dev/js/spec/buttongroupSpec.js
+++ b/dev/js/spec/buttongroupSpec.js
@@ -74,7 +74,7 @@
 
       var count = 0;
 
-      group.add('Meta', undefined, function () {
+      let btn = group.add('Meta', undefined, function () {
         count++;
       });
 
@@ -84,6 +84,10 @@
       group.element().firstChild.click();
 
       expect(count).toEqual(1);
+
+      expect(btn.textContent).toEqual("Meta");
+      btn.changeTitle("Cool");
+      expect(btn.textContent).toEqual("Cool");
     });
 
     it('should respect binds', function () {
diff --git a/dev/js/src/buttongroup.js b/dev/js/src/buttongroup.js
index da12078..40eb61f 100644
--- a/dev/js/src/buttongroup.js
+++ b/dev/js/src/buttongroup.js
@@ -115,7 +115,12 @@
       };
 
       b.setAttribute('title', desc);
-      b.addE('span').addT(title);
+      let innerSpan = b.addE('span');
+      innerSpan.addT(title);
+
+      b["changeTitle"] = function (title) {
+	innerSpan.textContent = title;
+      };
       
       let that = this;
       b.addEventListener('click', function (e) {