All cdkDrag items in a CdkDrop element are assumed to be members of a reorderable list

Currently all `cdkDrag` items in a `CdkDrop` element are treated as reorderable items in a list. While the underlying data structure is still left to the developer, the `CdkDragDrop` system updates the UI temporarily with changes to the "list order" even if the items are not supposed to be treated as a list.

Instead, we should have two directives for `CdkDrop` areas, `cdkDropZone` and `cdkDropList`. The former, does not do any automatic UI changes during the drag action and the latter does the current action of reordering items in the list.

Inizia tracciando gli entry point CdkDrop, CdkDragDrop, cdkDropZone e cdkDropList descritti nell’issue. Determina in che modo il comportamento attuale di drop aggiorna l’ordine della lista, quindi definisci i comportamenti separati in modo che cdkDropZone lasci invariato l’ordine della UI, mentre cdkDropList mantenga il riordinamento.

Scritto dal modello di indicizzazione a partire dal testo della issue.