Dropzone, added DragEnd property to allow executing logic when a drag operation ends

[ViRuSTriNiTy]

see #96

[Postlagerkarte]

Looks great! Thanks.

[ViRuSTriNiTy]

@Postlagerkarte Would you please release a new NuGet package? Otherwise I need to work with a git submodule and this is somewhat tedious.

[k2tomasz]

@ViRuSTriNiTy DragDropService is reset (DragDropService.ActiveItem = default) before DragEnd is called resulting in null being passed as parameter to DragEnd.

[ViRuSTriNiTy]

@tkrakowiak Can you provide a minimal reproducible example?

[k2tomasz]

@ViRuSTriNiTy you can change your ToDoList example like this:

<Dropzone Items="MyToDoList" DragEnd="DragEnd">
    <Todo OnRemoveClick="(item)=>MyToDoList.Remove(item)" Item="@context"></Todo>
</Dropzone>

private void DragEnd(TodoItem obj)
{
    var todo = obj;
}

[k2tomasz]

@ViRuSTriNiTy I think this PR would be enough to fix it.

#119