Add a button to clear the notification history

Summary:
Adds a button in the plasmoid header to clear the history, same as the context menu.
The icon used is the same as clear history in the klipper plasmoid.

BUG: 386068
FIXED-IN: 5.13.0

Test Plan: Please test the alignment on your screen, whilst I made sure to use provided units, due to the indentation of heading I had to use fractions, which might lead to odd results on different resolutions / dpi

Reviewers: broulik

Reviewed By: broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11261