Add #empty slot

[arthabus]

Alongside #header and #footer, add a slot to provide custom content when the list is empty.

This would allow to customize prompts for users when the list is empty while allowing to drag and drop from another list, for example:

<draggable>
    <template #empty>
        <div>Drop items here or click to create a new item</div>
    </template>
</draggable>

Kanban is a particular example for such functionality. Or in general anytime there is more than one list to move items between.