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

首先定位 Angular Material 的 expansion panel 组件及其现有的交互测试。检查当前如何处理对面板标题和箭头的点击，然后定义可选行为，并验证标题选择和箭头切换能够彼此独立地工作，同时不改变默认行为。

由索引模型根据 Issue 内容生成。