This makes the standalone KCM dialog consistent with the same KCM
when launched via kcmshell, systemsettings, or even other modules.
The dialog is required so it doesn't need to depend on kde-cli-tools.
This KCM will have to get installed installed in the new plasma namespace eventually.
But considering that this app is part of the release service, we should not
depend on changes in a not yet released Plasma version.
Task: https://phabricator.kde.org/T14501
The settings app is supposed to be used on its own. The buttons are without
any margins to the window border though (when borders are deactivated - the
default on Breeze).
Set the default layout margins to improve the look.
