Fix QML warnings due to depricated on<signal> calls in Connections

As of 5.12, the signals used in connections should not be declared as on<signal>, but as
function on<signal>(arguments).

plugins/PerObjectSettingsTool/PerObjectItem.qml

@@ -35,7 +35,7 @@ UM.TooltipArea
     Connections
     {
         target: addedSettingsModel
-        onVisibleCountChanged:
+        function onVisibleCountChanged()
         {
             check.checked = addedSettingsModel.getVisible(model.key)
         }