initial work on updating touch bar item config without rerender

lib/browser/api/touch-bar.js

@@ -44,6 +44,16 @@ class TouchBarItem {
     }
   }
 
+  updateConfig(newConfig) {
+    if (!this._owner) {
+      throw new Error('Cannot call methods on TouchBarItems without assigning to a BrowserWindow');
+    }
+    const dupConfig = Object.assign({}, newConfig);
+    delete dupConfig.id;
+    Object.assign(this.config, dupConfig);
+    this._owner._updateTouchBarItem(this.toJSON());
+  }
+
   toJSON () {
     return this.config
   }