Summary:
When we create a new Shellclient there may be a pending relevant
interface.

For every other case this is handled in WaylandServer, the class
responsible for attaching new interfaces at runtime.
The only exception is ServerSideDecorationInterface which is handled in
the ShellClient constructor.

This makes everything consistent.

No behavioural changes.

Test Plan: Ran unit tests

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18592