[klipper] Move notification from tray to Klipper

Summary:
Klipper shows a notification when the shortcuts next/prev history item
gets triggered. This notification used to be implemented in the class
KlipperTray. With the switch from SNI based clipper to a Plasmoid the
notification got lost as the Plasmoid doesn't use the KlipperTray class
at all.

This change moves the notification handling from KlipperTray to Klipper
so that it gets emitted for both SNI and Plasmoid based klipper.

BUG: 368808
FIXED-IN: 5.8.1

Test Plan: Set the shortcuts, triggered them and verified the notification gets shown.

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2942