Use geometry conversion helpers from AbstractClient in InternalClient

Summary:
mapFromClient and mapToClient are equivalent to clientRectToFrameRect and
frameRectToClientRect, respectively.

Test Plan: Compiles.

Reviewers: #kwin, romangg

Reviewed By: #kwin, romangg

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D26508