lookandfeel kcm: port from KRun::runApplication to KIO::ApplicationLauncherJob

Summary:
I used the dialog ui delegate because this is a KCM, even though it
seems to use QtQuick itself.

Test Plan:
Builds. `kcmshell5 lookandfeel` seems to add stuff to autostart
only for some themes I don't have (commit 49ae81f2 talks about lattedock
or conky).

Reviewers: broulik, ngraham, crossi, mart, davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29125