Housekeeping: Clear old notifications

### Problem

Notifications pile up across reboots in the indicator, forcing to manually clear them to avoid clutter
If other OS elements are cleared after a while, it makes sense to clear notifications as well.

### Proposal

Add a checkbox allowing to delete old notifications.

### Prior Art (Optional)

If i have a minute or two ill do a design for this, but it would pretty much follow the rest.
A checkbox along with "Delete automatically"

The text would read: "Notifications in Wingpanel"

The below text would be amended to say "Delete obsolete files _and notifications_ after:"