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

Beginne damit, die Angular Material expansion panel-Komponente und ihre bestehenden Interaktionstests zu lokalisieren. Prüfe, wie Klicks auf den Panel-Titel und den Pfeil derzeit behandelt werden, definiere anschließend das optionale Verhalten und verifiziere, dass die Titelauswahl und das Umschalten des Pfeils unabhängig voneinander funktionieren, ohne das Standardverhalten zu ändern.

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