Refactor foreach, use Qt5 signal syntax, remove unnecessary method call

Summary:
The two foreach occurences are refactored and the signals in the runner/kcm now use the C++ 11 for loop.
Additonally the duplicate call of the load method has been removed and smaller improvements have been made.

Test Plan: Compile time check for signals, trying out the runner and editing the config.

Reviewers: #plasma, ngraham, broulik, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27070