Summary:
KWin's plugins are now all in a plugins subdirectory. This is a good
argument to also move the window decoration plugins there. The name
clients was not really good anyway and makes it difficult for people
not familiar with the code base to find it. Having it under plugins
is the more expectable location.
Reviewers: #plasma
Subscribers: plasma-devel
Projects: #plasma
Differential Revision: https://phabricator.kde.org/D1414
