title: How to prevent toggling mat-expansion-panel by clicking mat-expansion-panel-header?
body: <p>I know mat-expansion-panel-headers is a button. clicking anywhere on that button toggles the expansion opens/closes. But I don't want to let users click anywhere on the header and open it. There should be a small button. One click on that button will open/close the expansion panel. How can I do that?</p>
  
  <p>I have tried this, but it didn't work.</p>
  
  <pre><code>&lt;mat-expansion-panel&gt;
  &lt;mat-expansion-panel-header (click)="$event.preventDefault()"&gt;
    &lt;mat-panel-title&gt;
      MENU
    &lt;/mat-panel-title&gt;
  &lt;/mat-expansion-panel-header&gt;
  </code></pre>
  
  <p></p>