Allow to undo the clear actions on the playlist

Summary:
When a clear action is performed on the playlist, the user might lose valuable work.
Add the possibility to undo this operation via passive notification.
Keep it simple by allowing only one undo and no redo.

• The patch now enables to restore the playlist and the player status.

For the notification, I use KNotification here, you can see in the screenshot the notification with an Undo button.

• KNotification needs the file elisa.notifyrc to be installed in QStandardPaths::GenericDataLocation directory, so it may impact the packaging of the app (https://api.kde.org/frameworks/knotifications/html/classKNotification.html#file).

Reviewers: mgallien

Reviewed By: mgallien

Subscribers: ngraham

Tags: Elisa

Maniphest Tasks: T5376

Differential Revision: https://phabricator.kde.org/D19610