title: Why is the ion-back-button not shown?
body: <p>The <code>ion-back-button</code> does NOT show up to the right of the <code>ion-menu-button</code>. Why is that?</p>
  
  <p>the <code>ion-menu-button</code> and the <code>ion-title</code> show properly and aligned on the same horizantal position.</p>
  
  <pre><code>&lt;ion-header&gt;
    &lt;ion-toolbar&gt;
  
      &lt;ion-buttons slot="start"&gt;
        &lt;!-- navigation button--&gt;
        &lt;ion-menu-button&gt;&lt;/ion-menu-button&gt;
        &lt;!-- optional back button--&gt;
        &lt;ion-back-button&gt;&lt;/ion-back-button&gt; 
      &lt;/ion-buttons&gt;
  
      &lt;ion-title&gt;
          {{ pageTitle | translate}}
      &lt;/ion-title&gt;
  
    &lt;/ion-toolbar&gt;
  &lt;/ion-header&gt;
  </code></pre>
  
  <p>In the DOM inspector the CSS <code>display</code> attribute of the <code>ion-back-button</code> is set to <code>none</code>. Why would it set itself to <code>none</code>? </p>
  
  <p>I used </p>
  
  <pre><code>this.navCtrl.navigateForward('/term/' + term);
  </code></pre>
  
  <p>to navigate to this page, thus I expect the back button to pick this up. Why is <code>navigateForward</code> not adding to the stack, which would make the <code>ion-back-button</code> show?</p>