Use sdk/panel onOpen event listener

When the panel is constructed, Recommend.js's handlePanelOpen is specified as the callback function for whenever the panel opens. The telemetry logging for panel open events is done here. This way, Recommend.js does not have to keep track of the state of the panel.

Closes #66

lib/Recommend.js

@@ -19,7 +19,7 @@ class Recommender {
   constructor() {
     this.telemetryLog = new TelemetryLog();
     const methodsToBind = ['endOnboard', 'showPanel', 'waitForWindow',
-                           'handlePanelHide', 'hidePanel'];
+                           'handlePanelHide', 'handlePanelShow', 'hidePanel'];
     for (let key of methodsToBind) { // eslint-disable-line prefer-const
       this[key] = this[key].bind(this);
     }
@@ -72,18 +72,22 @@ class Recommender {
       height: panelRecHeight + footerHeight,
       contentURL: './recommendation/recWindow.html',
       contentScriptFile: './recommendation/notify.js',
+      onShow: this.handlePanelShow,
       onHide: this.handlePanelHide,
     });
   }
 
   showPanel(button) {
-    this.telemetryLog.showPanel(this.activeRecDomain);
     button.handlePanelOpen();
     this.panel.show({
       position: button,
     });
   }
 
+  handlePanelShow() {
+    this.telemetryLog.showPanel(this.activeRecDomain);
+  }
+
   handlePanelHide() {
     this.telemetryLog.hidePanel(this.activeRecDomain);
     const win = tabs.activeTab.window;