Generic Drag&Drop get ctrl, shift, alt and meta key

[probert94]

Description of the feature

I am using the generic Drag&Drop and would like to be able to change the behavior based on the pressed key modifiers. Example: In Windows, draging a file usually moves it, but if you press ctrl, it will copy the file instead. I would like to be able to do the same in Vaadin.

Note:

It seems to be enough to add @EventData("event.ctrlKey") boolean ctrlKey and so on to the constructor of DragEvent (and maybe also the other Drag&Drop related events) and store the result in a member.
For now, I created a custom DragEvent which adds the given information.

Versions:

- Vaadin / Flow version: 19.0.6