Add a way to dismiss all error pop-up notificaions

Is your feature request related to a problem? Please describe.
I copied a folder with several hundred pictures to a different folder that already contained most but not all of those pictures. Thus, I got several hundred "Could not copy ..., target exists" pop-ups:

So far, those need to be dismissed one by one as they do not seem to disappear after a while. As a workaround I found that they are cleared when I reload the site, though that's not very user friendly.

Describe the solution you'd like
It would be nice to have way to dismiss all of these pop-ups, potentially with a "Dismiss all" appearing above when more than one pop-up is displayed.

Describe alternatives you've considered
Another possibility would be to let these pop-ups vanish after a short amount of time. However if they are actually displaying important information, it can be frustrating when the error message disappears before you got the chance to copy it.

Start by locating the notification or error pop-up implementation in the nextcloud-dialogs repository and reproduce the multi-error case described in the issue. Trace how individual pop-ups are dismissed, then add a way to dismiss all displayed errors while preserving the existing individual-dismiss behavior. Done means several persistent error notifications can be cleared together without reloading the site.