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.

Commencez par examiner les API Angular CDK de glisser-déposer et le comportement décrit pour les éléments cdkDrag autonomes. Le travail est terminé lorsqu’un cdkDrag situé en dehors d’une liste de dépôt peut notifier les consommateurs lorsque le glissement commence et lorsqu’il est déposé sur une cible arbitraire.

Rédigé par le modèle d'indexation à partir du texte de l'issue.