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