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/MachineSettingsAction/MachineSettingsPrinterTab.qml

@@ -326,7 +326,7 @@ Item
                 Connections
                 {
                     target: Cura.MachineManager
-                    onGlobalContainerChanged: extruderCountModel.update()
+                    function onGlobalContainerChanged() { extruderCountModel.update() }
                 }
             }