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();
+                    }
                 }
             }
 

resources/qml/MachineSettings/ComboBoxWithOptions.qml

@@ -72,23 +72,16 @@ UM.TooltipArea
                 return;
             }
 
-            if (typeof propertyProvider.properties["options"] === "string") {
+            if (typeof(propertyProvider.properties["options"]) === "string") {
                 return;
             }
 
-            let currentIndex = -1;
             const keys = propertyProvider.properties["options"].keys();
             for (let index = 0; index < propertyProvider.properties["options"].keys().length; index ++) {
                 const key = propertyProvider.properties["options"].keys()[index];
                 const value = propertyProvider.properties["options"][key];
-
-                if (propertyProvider.properties.value === key) {
-                    currentIndex = index;
-                }
                 defaultOptionsModel.append({ text: value, value: key });
             }
-
-            comboBox.currentIndex = currentIndex;
         }
 
         Component.onCompleted: updateModel()
@@ -112,7 +105,7 @@ UM.TooltipArea
         model: defaultOptionsModel
         textRole: "text"
 
-        currentIndex: function () {
+        currentIndex: {
             const currentValue = propertyProvider.properties.value
             for (let i = 0; i < model.count; i ++) {
                 if (model.get(i).value === currentValue) {
@@ -123,7 +116,6 @@ UM.TooltipArea
         }
 
         onActivated: function (index) {
-            const key = propertyProvider.properties["options"].keys()[index];
             const newValue = model.get(index).value;
 
             if (propertyProvider.properties.value !== newValue && newValue !== undefined) {