kcmshell: Remove dbus registration of modules, cleanups and refactorings

This is quite problematic, because in case modules are already launched, it will print a message on the console and wait for the other instance to finish. Also, the registration was fragile, because in case multiple KCMs are loaded in a different order, the servicename would be different. Meaning if you launch the KCMs "icons" and "colors" in one instance and then "colors" and "icons" in another one, you would have to kcmshell instances displaying the same KCMs.

Because we use systemsettings (which is a single-instance app) for most KCMs, it is pretty unlikely that two KCMs are opened at the same time anyway.