refactor: Minimize use of geom in Toplevel subclasses

Summary:
This change makes easier to refactor geometry handling in the future.
The main motivation for avoiding using geom directly is to make code
more readable and ensure that the geometry is updated only through
designated methods, e.g. setGeometry, plainResize, etc.

Reviewers: #kwin, romangg

Reviewed By: #kwin, romangg

Subscribers: romangg, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D23072