Simplify code dealing with creating new sessions.

Summary:
Remove duplicated code in various places dealing
with starting new sessions.

Test Plan: open konsole, multiple tabs, closed, didn't crash, seems to work.

Reviewers: Konsole, hindenburg

Reviewed By: Konsole, hindenburg

Subscribers: konsole-devel

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D12968