wallpaper configuration separed by plugin

don't let different plugins step on each other toes when configuring, if i
get back to an old plugin, it gets correctly restored where i was

src/scriptengines/qml/plasmoid/wallpaperinterface.cpp

@@ -98,6 +98,7 @@ KConfigLoader *WallpaperInterface::configScheme()
 
         KConfigGroup cfg = m_containmentInterface->containment()->config();
         cfg = KConfigGroup(&cfg, "Wallpaper");
+        cfg = KConfigGroup(&cfg, m_wallpaperPlugin);
 
         if (xmlPath.isEmpty()) {
             m_configLoader = new KConfigLoader(cfg, 0, this);