Replace old notification when new notification for connection appears

Summary:
When a second notification appears shortly after another, e.g. a wifi network disconnects and then reconnects, replace the first notification with the second one. This avoids having a disconnected and a connected notification simultaneously.

Depends on D28691

Test Plan: dis- and reconnected my wifi

Reviewers: #plasma, broulik, jgrulich

Reviewed By: jgrulich

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28693