Simplify notification handling

Summary:
Use QPointer for KNotification
Use ready signal for signalling updates

BUG: 400010

Test Plan: Spawned some notifications

Reviewers: KDE Connect, broulik, albertvaka

Reviewed By: KDE Connect, albertvaka

Subscribers: albertvaka, kdeconnect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D18354