[applets/notifications] Move "Clear History" action out of burger menu
The fact that the commonly-used "Clear History" action appears in a hamburger menu has been a common user complaint about Plasma 5.21.
This happens because it's defined as an action, and the applet has two actions ("clear history" and "open KCM") and when there is more than one action, they all go into a hamburger menu. This makes the commonly-used clear action more difficult to find that necessary.
A reasonable long-term solution would be to refactor the action visibility code in plasma-framework to allow the display of more than one action in the header without always putting them all in a hamburger menu, or allowing the developer more control over this. However since that would require frameworks changes, it can't be done in Plasma 5.22 because 5.22 requires Frameworks 5.82 which has already been tagged. So let's do this simpler fix for now to fix the UX regression.
BUG: 433140 FIXED-IN: 5.22
