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).

resources/qml/WelcomePages/AddPrinterByIpContent.qml

@@ -41,7 +41,7 @@ Item
     Connections
     {
         target: CuraApplication.getDiscoveredPrintersModel()
-        onDiscoveredPrintersChanged:
+        function onDiscoveredPrintersChanged()
         {
             if (hasRequestFinished && currentRequestAddress)
             {
@@ -310,7 +310,7 @@ Item
                     Connections
                     {
                         target: CuraApplication.getDiscoveredPrintersModel()
-                        onManualDeviceRequestFinished:
+                        function onManualDeviceRequestFinished(success)
                         {
                             var discovered_printers_model = CuraApplication.getDiscoveredPrintersModel()
                             var printer = discovered_printers_model.discoveredPrintersByAddress[hostnameField.text]

resources/qml/WelcomePages/DropDownWidget.qml

@@ -35,7 +35,7 @@ Item
     Connections
     {
         target: header
-        onClicked:
+        function onClicked()
         {
             base.contentShown = !base.contentShown
             clicked()

resources/qml/WelcomePages/FirstStartMachineActionsContent.qml

@@ -27,7 +27,7 @@ Item
     Connections
     {
         target: machineActionsModel
-        onAllFinished:
+        function onAllFinished()
         {
             if (visible)
             {

resources/qml/WelcomePages/WelcomeDialogItem.qml

@@ -61,6 +61,6 @@ Item
     Connections
     {
         target: model
-        onAllFinished: dialog.visible = false
+        function onAllFinished() { dialog.visible = false }
     }
 }

resources/qml/WelcomePages/WizardDialog.qml

@@ -47,6 +47,6 @@ Window
     Connections
     {
         target: model
-        onAllFinished: dialog.hide()
+        function onAllFinished() { dialog.hide() }
     }
 }