Summary:
in prevision of more or less replacing kcmshell with systemsettings
itself, suppo loading a module as a paramenter as well as a --list
paramenter

Test Plan:
systemsettings5 kwindecoration loads the decoration kcm and the proper
category selected on the sidebar

Reviewers: #plasma, ngraham

Reviewed By: ngraham

Subscribers: broulik, bport, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29064