[kcm] Update device combobox when current device changes externally

Otherwise changing the device from e.g. the applet doesn't update the
running KCM


(cherry picked from commit 1729c76d)

src/kcm/package/contents/ui/DeviceComboBox.qml

@@ -13,7 +13,13 @@ ComboBox {
     textRole: "Description"
     valueRole: "Index"
 
+    property int deviceIndex: DeviceIndex
+
     onActivated: DeviceIndex = currentValue
 
+    onDeviceIndexChanged: {
+        currentIndex = indexOfValue(DeviceIndex)
+    }
+
     Component.onCompleted: currentIndex = indexOfValue(DeviceIndex)
 }