Allow Drag and Drop from/to arbitrary areas

#### Bug, feature request, or proposal:
Dragging and dropping in a cdkDropList is nice, but we need the ability to set an element as a cdkDrag element, and be able to drag that element onto an arbitrary drop zone/target. Currently, there is no way to know if a cdkDrag that is not part of a drop list is dragged - there aren't any "drag start", "drag end/dropped" events.

#### What is the expected behavior?
Be able to be notified when a cdkDrag element that is not within a drop list is dragged and dropped.

#### What is the current behavior?
Not the above.

#### What is the use-case or motivation for changing an existing behavior?
Be able to drag elements from a list / grid onto a target area, not necessarily moving the item, but to at least be notified when it is dropped into the target, in order to do something (play a sound or animation, etc.)

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular 7.0.2
Material 7.0.2
All browsers.

まず、Angular CDK のドラッグ＆ドロップ API と、スタンドアロンの cdkDrag 要素について説明されている動作を確認します。ドロップリスト外の cdkDrag が、ドラッグの開始時と任意のターゲットへのドロップ時にコンシューマーへ通知できるようになれば、作業は完了です。

索引モデルが issue の本文から書いたものです。