Implement the shortcut caption suffix for Wayland windows

Summary:
The generation of the shortcut caption part is moved from Client to
AbstractClient. The ShellClient also has a captionSuffix and implements
the full part in caption.

Overall this needs more refactoring to support more sharing between the
two implementations. But one step at a time.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D7093