The IDL constructor has to take separate steps than a DataTransfer that
is internally constructed. Notably, an IDL-created object has its own
drag data store, and that store is placed in a read-write mode.
Ownership of the drag data store is a bit weird. In a normal drag-and-
drop operation, the DragAndDropEventHandler owns the store. When events
are fired for the operation, the DataTransfer object assigned to those
events are "associated" with the store. We currently represent that with
an Optional<DragDataStore&>.
However, it's also possible to create DataTransfer objects from scripts.
Those objects create their own drag data store. This puts DataTransfer
in a weird situation where it may own a store or just reference one.
Rather than coming up with something like Variant<DDS, DDS&> or using
MaybeOwned<DDS> here, we can get by with just making the store reference
counted.
The drag-and-drop processing model allows for users to drag around
either elements within the DOM or objects completely outside the DOM.
This drag event can either end without action (via cancellation or user
input), or in a drop event, where the dragged object is dropped onto
another element within the DOM.
The processing model is rather large. This implements enough of it to
allow the UI process to specifically handle dragging objects outside of
the DOM onto the DOM. For example, dragging an image from the OS file
manager onto a file-upload input element. This does not implement the
ability to drag DOM elements.
