title: Ionic2 ion-slides change navbar title
body: <p>I'm trying to change the navbar title using the slides.
  When swiping through a next slide, how can I achieve this?
  I tried:</p>
  
  <pre><code>onSlideChangeStart(slider) {
  this.app.setTitle('title1');
  }
  </code></pre>
  
  <p>But it just sets the title in the browser, not in the ion-title tag.
  And I want to set the title according to the slide that is displaying.</p>
  
  <p>This is my code:</p>
  
  <pre><code>&lt;ion-header&gt;
  &lt;ion-navbar color="navh" no-border-bottom&gt;
  &lt;ion-title&gt;title&lt;/ion-title&gt;
  &lt;/ion-navbar&gt;
  &lt;/ion-header&gt;
  &lt;ion-content  no-bounce &gt;
  &lt;ion-slides [options]="mySlideOptions (ionWillChange)="onSlideChangeStart($event)"&gt;
  
  &lt;ion-slide&gt;
       &lt;img #title1 src="assets/img/1.png" &gt;
  &lt;/ion-slide&gt;
  &lt;ion-slide&gt;
       &lt;img #title2 src="assets/img/2.png" &gt;
  &lt;/ion-slide&gt;
  &lt;ion-slide&gt;
       &lt;img #title3 src="assets/img/3.png" &gt;
  &lt;/ion-slide&gt;
  &lt;/ion-slides&gt;
  &lt;/ion-content&gt;
  </code></pre>