automatically hide undo notification 3 seconds after it has been shown

Summary:
A timer is used to automatically hide the notification 3 seconds after it is being shown.

The duration is currently not user configurable.

BUG: 406603

Test Plan: 3 seconds after it is being shown, the notification is automatically removed

Reviewers: Elisa, ngraham

Reviewed By: ngraham

Subscribers: shubham, pino

Differential Revision: https://phabricator.kde.org/D20670