add the ability for kded modules to have kcms

this uses the established tech of kcmservices that can "associate" with a kplugininfo and we only need to display them

somewhat unfortunately this pulls in qwidgets because we don't seem to have a qml variant of KCMultiDialog, not that it matters greatly anyway since we need to embed both qml and qwidget kcms inside the dialog.

this would be used by the kcm setting X-KDE-ParentComponents=freespacenotifier for example