SessionManager: Avoid to accumulate outdated entries in the session file

Summary:
Because the various save routines writes always only actual data to the
session file remain data from e.g. closed documents, views or splitter which
is at least ugly and a waste of expensive disk space :-)

Test Plan: Take a look in your session files, typical at ~/.local/share/kate/sessions/ you will notice a bunch of file hints you didn't have open/loaded

Reviewers: #kate, cullmann

Reviewed By: #kate, cullmann

Subscribers: cullmann, kwrite-devel, #kate

Tags: #kate

Differential Revision: https://phabricator.kde.org/D16687