optionally show a history of notifications

Summary:
Bug 378032.
	With these changes, notifications can be configured to always persist in the notifications applet.

	Todo: filter by application, see https://bugs.kde.org/show_bug.cgi?id=378032#c27

Test Plan: Open the notification plasmoid's settings. Check 'Show a history of notifications'. Trigger some notifications. They should show up in the applet.

Reviewers: #plasma, #vdg, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: mart, graesslin, davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D7271