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