[scripting] Introduce writable frameGeometry property

Summary:
The geometry property has been deprecated for quite some time and its
usage in scripts is highly discouraged. Since AbstractClient overrides
the geometry property to make it writable, the same thing must be done
for the frameGeometry property.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D29666