MessageInterface: Add CenterInView as additional position

Summary:
This patch extends the message interface to support CenterInView.
This should be binary compatible, since an enum is only added at
the end. In order to support this, a custom layout called
KateMessageLayout was added that only supports the floating
positions allowed by KTextEditor::Message::MessagePosition.

Upadte unit test, it seems the window size was too small for
the MessageTest.

Test Plan: make test

Reviewers: cullmann

Reviewed By: cullmann

Subscribers: Frameworks

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D9393