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

Start by locating the Angular Material expansion panel component and its existing interaction tests. Review how clicks on the panel title and arrow are currently handled, then define the optional behavior and verify that title selection and arrow toggling work independently without changing the default behavior.

Written by the indexing model from the issue text.