Make session loading/saving helpers local to sm.cpp

Summary:
There is no point for keeping them in the Workspace class because they
are used only in sm.cpp.

Test Plan: Compiles.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D20783