feat(cdkDropList): Add configuration input to prevent list rearranging

[dpsthree]

Bug, feature request, or proposal:

Add an input property to cdkDropList directive that prevents the draggable elements within it from rearranging. For example:
@Input() preventRearrange: boolean = false

What is the expected behavior?

When preventRearrange is set to true the other elements within the drop list would remain in the initial position while the user is dragging an element. When set to false, cdkDropList would behave as it does now.

What is the current behavior?

There is currently no way to prevent list items from rearranging

What is the use-case or motivation for changing an existing behavior?

This is useful for features in which the user is expected to move items between lists, but the order is determined by sources other than the user. For example, if two lists are supposed to remain in alphabetical order, but users can move items from one list to the other.

This would also make it easier to develop features where the item being dragged is not removed from the source list. For example, a tool pallet that allows for the dragging and dropping of the same item multiple times.

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

7+

[dpsthree]

Alternatively, the implementation of arbitrary from/to without a dropList could also solve this problem.

[princemaple]

This is also useful for dragging from an area that the draggable items span multiple rows/columns. currently it just behaves terribly because reordering only supports either row or column, not grids, etc.

[princemaple]

@crisbeto looks like this one can be closed since your #15064 is in.

[crisbeto]

Thanks for the heads-up. Closing.

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