Be explicit on what the ViewContainer accepts

Summary:
This is needed to clean up a bit of the code later.
being really explicit here removes the need to handle
QWidget pointers, as that could in theory be anything

Handling only TerminalDisplays* we can remove the
SessionController map from the ViewContainer and
add it to the TerminalDisplay class in the future

Reviewers: Konsole, hindenburg

Reviewed By: Konsole, hindenburg

Subscribers: konsole-devel

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D17492