Rename Toplevel::geom to Toplevel::m_frameGeometry

Summary: The new name better reflects what Toplevel::geom is.

Test Plan: Compiles, tests still pass.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D25738