Fix extruder number

most important change was changing
```
        currentIndex: function () {
          ...
        }
```
to
        currentIndex: {
          ...
        }
```
This changed the behavior so that the variables inside the code blocked are watched so that `currentIndex` is automatically updated. No longer needed to set the `currentIndex` from any where else anymore.

CURA-10374

plugins/MachineSettingsAction/MachineSettingsPrinterTab.qml

@@ -303,18 +303,17 @@ Item
 
                     Component.onCompleted:
                     {
-                        update()
+                        updateModel();
                     }
 
-                    function update()
+                    function updateModel()
                     {
-                        clear()
-                        for (var i = 1; i <= Cura.MachineManager.activeMachine.maxExtruderCount; i++)
-                        {
+                        clear();
+                        for (var i = 1; i <= Cura.MachineManager.activeMachine.maxExtruderCount; i ++) {
                             // Use String as value. JavaScript only has Number. PropertyProvider.setPropertyValue()
                             // takes a QVariant as value, and Number gets translated into a float. This will cause problem
                             // for integer settings such as "Number of Extruders".
-                            append({ text: String(i), value: String(i) })
+                            append({ text: String(i), value: String(i) });
                         }
                     }
                 }
@@ -322,7 +321,9 @@ Item
                 Connections
                 {
                     target: Cura.MachineManager
-                    function onGlobalContainerChanged() { extruderCountModel.update() }
+                    function onGlobalContainerChanged() {
+                        extruderCountModel.updateModel();
+                    }
                 }
             }