Summary:
We have duplicated code in InternalClient and XdgShellClient to create
decorations. In order to get rid of the code duplication, this change
introduces a method that AbstractClient subclasses can call to create
a window decoration.

Test Plan: Tests pass.

Reviewers: #kwin, apol

Reviewed By: apol

Subscribers: apol, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D27822