Add an explicit dependency to Breeze to find decoration plugin

Summary:
KWin needs the plugin id of the breeze decoration plugin. Instead
of hard coding that it's now resolved through an optional dependency.
If the optional dependency is not available, the default is adjusted
to aurorae/plastik.

Reviewers: #plasma

Subscribers: plasma-devel

Projects: #plasma

Differential Revision: https://phabricator.kde.org/D1344