ViewDocumentDragStartEvent
typedef
Fired when the user starts dragging the content in one of the editing roots of the editor.
Introduced by ClipboardObserver.
Note: This event is not available by default. To make it available, ClipboardObserver
needs to be added to the ViewDocument by using the
addObserver method. This is usually done by the Clipboard
plugin, but if for some reason it is not loaded, the observer must be added manually.