This patch extends the message interface to support CenterInView.
This should be binary compatible, since an enum is only added at
the end.
The search messages "Search wrapped at bottom/top" now use
CenterInView to be more visible (see recent discussion on
kwrite-devel). In order to support his, a custom layout called
KateMessageLayout was added that only supports the positions
allowed by KTextEditor::Messate::MessagePosition.