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

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

[javier-godoy]

See #7500

[probert94]

I just noticed, that a few things (like the v-drag-over-target class) are linked to the DropEffect you set on the DropTarget. So solving the related Issue #7500 will probably not be that easy. So I think it wouldn’t be a bad idea to add the KeyModifiers (the related flags) to the dnd events, as it won’t hurt if they are not needed. It’s just an additional information you can use if you want.