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

まず Angular Material の expansion panel コンポーネントと既存のインタラクションテストを見つけます。パネルのタイトルと矢印のクリックが現在どのように処理されているかを確認し、次にオプションの動作を定義して、タイトルの選択と矢印の切り替えがデフォルトの動作を変更せずに独立して機能することを検証します。

索引モデルが issue の本文から書いたものです。