Simplify saveShortcuts and saveFavorites

Summary:
Great part of the code inside saveShortcuts and saveFavorites
is similar, extract it to a new function and call it.

Test Plan: Edited shortcuts, saved, saves where correct.

Reviewers: hindenburg, Konsole

Reviewed By: hindenburg, Konsole

Subscribers: konsole-devel

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D13539