Support drag and drop to tree elements.

#### Bug, feature request, or proposal:
Feature request

#### What is the expected behavior?
Currently the d&d behavior allows to drag items from 1 **cdkDropList** to another **cdkDropList**

In my case I need to copy (by drag & drop) item (row) from a table to a tree.

I tried to use the cdk but handle a few problem.

#### What is the current behavior?
1. (This is a major problem) - I can't seem to work with **cdkDropList** inside a tree, since it has an hierarchical structure , the item is always dropped in the root node, **How can I drop the item into a child node???**
There is also a problem with the **cdkDropListConnectedTo** since I don't want to give the id's of each node in the tree.

2. There is no source container - I put a **cdkDropList** directive in my table element (which I drag items from) and it gave me the ability to drag rows from the table to the tree, as oppose to drag the rows freely on the screen. This is great.
However, it gave me a behavior that I don't wan't, re order items inside the table: When I drag over the table, the items move, but since it just a source container to copy items from they really need to be fixed, And it will be great if there was a way to fix the dragged item itself (Apply copy behavior using the preview image instead of a move behavior).

**Update** I think issue #13100 discuss this feature in depth.

3. ( more of a proposal) - In my *cdkDragPreview I render a label with all the information I need, so when I try to drop the item, There is the preview as well as the placeHolder image, I think it would be great to allow displaying only one of the two).

#### What are the steps to reproduce?
- Add a table with cdkDropList
- Add to the table some rows with cdkDrag
- Add at tree element (for each node apply the cdkDropList directive).
- try to drag element from the table and drop it into a child drop list.

#### What is the use-case or motivation for changing an existing behavior?
As I said, I need to **copy** items from a **mat-table** to **mat-tree**

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

Angular CLI: 7.0.2
Node: 9.0.0
OS: win32 x64
Angular: 7.0.0
... compiler, core

Package Version
------------------------------------------------------
@angular-devkit/architect 0.10.2
@angular-devkit/core 7.0.2
@angular-devkit/schematics 7.0.2
@angular/cli 7.0.2
@schematics/angular 7.0.2
@schematics/update 0.10.2
rxjs 6.3.3
typescript 3.1.3
material 7.0.1

#### Is there anything else we should know?

Start by reviewing the Angular CDK cdkDropList and cdkDrag behavior alongside the mat-table and mat-tree usage described in the reproduction steps. Read issue #13100 for the related discussion, then separate the requested tree-child drops, source-container copy behavior, and preview/placeholder behavior; done means the intended interactions are specified and supported.

Written by the indexing model from the issue text.