kcms: Port away from org.kde.kcm.ConfigModule

org.kde.kcm.ConfigModule doesn't exist anymore. Its replacement lives in
org.kde.kcmutils module now.

Note that the replacement has no "quickHelp" property, so it's been
removed. While on this, also drop "ConfigModule.buttons". It still
exists in the new ConfigModule but the default buttons should be fine.