[Notifications] Move out of the way of dialog rather than just hiding

When e.g. connecting to a network with captive portal, it leaves you sitting there wondering what's going on.
The notification won't time out until it becomes visible again but still there's been complaints that the notifications go missing.
This patch makes them move out of the way vertically.
It is pretty crude but covers the majority ("System Tray covers notifications") case.

Differential Revision: https://phabricator.kde.org/D25566