[Feature] Undo closing a notification

[bakhtiyarneyman]

Is your feature request related to a problem? Please describe.
Very often, I accidentally close a notification and then am left guessing what it was. This is severely exacerbated by the fact that the del key currently closes the whole group.

Describe the solution you'd like
I would like to be able to bring back the closed notifications, one at a time, similarly to how dunst handles this. I think backspace is a good keybinding for this.

Describe alternatives you've considered

• Show all the closed notifications always in the Control Center, but grayed out.
• [Feature] Configure swaync-client --count to report non-hidden notifications. #508

[ArijanJ]

Related: #299