title: How to change toggle icon of mat-expansion-panel?
body: <p>The default icon for toggling a <code>mat-expansion-panel</code> is <code>&gt;</code>. Setting <code>hideToggle</code> true just hides the toggle icon. Is there any way to change it? I found nothing in the official documentation. I want to use <code>+</code> or <code>-</code> icon if the state is closed or opened respectively.</p>