This is a little sneak peek of a new feature that will arrive in Plasma 5.2 (OK, to be pedantic since frameworks has a separate schedule, you will be able to get it already next frameworks release a bit before Plasma’s)
Ever ended up deleting a desktop widget, panel widget or even an entire panel just to wish you hadn’t?
One solution may be littering everything with confirmation dialogs, but this quickly becomes annoying, besides the user becoming quickly trained to click “yes” without ever thinking about (I fallen too many times at this kind of dialogs).
So, what can be a better solution? Undo!

When the user deletes something, a notification pops up, notifying that an important thing has been deleted: it offers an action to undo the deletion.
If you click “undo” the widget or panel will instantly come back, if you manually close the notification, it will be irrevocably deleted.