Konsole: Use KMessageWidget in TerminalDisplay

Summary:
Depends on D10862

Before:

After:

Read-only:

Both:

This also fixes the overlapped scrollbar (see first screenshot).

Test Plan: Tested before and after, behaviour is the same.

Reviewers: Konsole, hindenburg

Reviewed By: Konsole, hindenburg

Subscribers: herrold, hindenburg, ngraham

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D10935