Support multiple stacked Passive Notifications

Summary:
add the ability to support more (up to an arbitrary 4) passive notifications shown at once
see also D28130 D28131

Test Plan:
it looks and behaves the same when there is only one. when there are multiple they are stacked on top of each other with the most recent at the top

Reviewers: Kirigami, broulik

Reviewed By: broulik

Subscribers: broulik, ngraham, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D28103