Add a button to clear the notification history
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.
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
Reviewed By: broulik
Differential Revision: https://phabricator.kde.org/D11261