Feature request - expansion panel option to only expand/collapse on ▽ or △

#### Bug, feature request, or proposal:

Feature request

#### What is the expected behavior?

Ability to optionally only open / close expansion panel when up/down arrow is selected rather than the entire panel title

#### What is the current behavior?

The entire title triggers the behaviour

#### What is the use-case or motivation for changing an existing behavior?

e.g. Outlook.com user interface - selecting a 'folder' is an action (e.g. edit the folder) - toggling its slider-like display requires action on the up/down button. Seems to make sense from a user interaction point of view. This can't be reproduced with the material expansion panel. It's useful when selecting the title implies 'more information' on this panel.

#### Is there anything else we should know?

Workaround (untested) - add click handler to mat-panel-title and do not propagate the event

Comienza localizando el componente expansion panel de Angular Material y sus pruebas de interacción existentes. Revisa cómo se gestionan actualmente los clics en el título del panel y en la flecha; después, define el comportamiento opcional y verifica que la selección del título y la alternancia de la flecha funcionen de forma independiente sin cambiar el comportamiento predeterminado.

Escrito por el modelo de indexación a partir del texto del issue.