Replace obsolete QSignalMapper and qSort with non-obsolete code

Summary:
qSort and QSignalMapper are deprecated and spoil the compilation messages with warnings:

https://build.kde.org/job/Applications/job/ktouch/job/kf5-qt5%20FreeBSDQt5.13/10/warnings12Result/

This patch tries to get rid of these warnings.

Test Plan:
1. Compile (there should be no warnings with Qt 5.13).
2. Run KTouch, open the course and layout editor and create a course. The new item should be correctly added to the lists in the Editor and the main KTouch window. Delete the course.
3. Add a layout. Add a key. Resize the key. KTouch should behave as expected. Remove the layout.

Reviewers: #kde_edu, gottfried

Reviewed By: gottfried

Subscribers: kde-edu

Tags: #kde_edu

Differential Revision: https://phabricator.kde.org/D22910