Don't set button text when reusing existing button elements

Since changeset 4744202e7c6a button elements are reused, so we must
only add its text when it does not exist otherwise the label is shown
multiple times.
issue5870

review32571002

src/common.js

@@ -720,17 +720,17 @@
                 this.el = el;
             } else {
                 this.el = jQuery('<button/>');
+                this.el.append(attributes.string || '');
             }
             this.icon = this.el.children('img');
             if (!this.icon.length) {
-                this.icon = jQuery('<img/>').appendTo(this.el);
+                this.icon = jQuery('<img/>').prependTo(this.el);
                 this.icon.hide();
             }
             this.el.addClass('btn btn-default');
             this.el.attr('type', 'button');
             this.icon.addClass('icon');
             this.icon.attr('aria-hidden', true);
-            this.el.append(attributes.string || '');
             this.set_icon(attributes.icon);
         },
         set_icon: function(icon_name) {