Use default close button of KMessageWidget

Summary:
D10935 introduced KMessageWidgets in TerminalDisplay for the "resume"
and "read-only" messages. This patch restores the default close button
and makes use of animateShow/animateHide. A nice plus is that I also
removed some code which has become obsolete AND that the messages
are now animated.

Test Plan:
Show/hide works like before.
Also still works if readonly and suspended at the same time.
Detaching works.

Reviewers: ach, Konsole, hindenburg

Reviewed By: Konsole, hindenburg

Subscribers: hindenburg, ach, Konsole

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D11241