Add a convenience property to retrieve the size of the decorated client

Summary:
It can be useful if we want to know both width and height or react to
any changes in the size of the decorated client.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D26937