This ensures that newer notification popups don't push old ones out.
Some applications are notorious for spamming the user which right now results in a firework
of notification popups. With this patch only the number that fits on screen is displayed and
everything beyond that is off screen until there's enough room for new popups which are
then gradually lowered.

It also reduces the likelihood of the notification the user is interacting with being shifted
away by an incoming notification. Furthermore, since notifications that are off screen will
have their timeout reset, it can happen that you have a bunch of recent notifications at the
bottom but then get some old ones that were off screen sliding back in because some middle ones
have expired already, leading to awkward results.

Notification scoring (e.g. critical before normal) is untouched, so a "battery is critical" notification
will show up even if it's still chugging through the backlog of browser notifications.

Differential Revision: https://phabricator.kde.org/D28646