Set the height of the stage menu in the theme instead of hardcoded.

I also adjusted a bit the design to fulfill the requirments from UX/UI
team.

Contributes to CURA-5772.

plugins/PrepareStage/PrepareMenu.qml

@@ -21,7 +21,6 @@ Item
 
     Row
     {
-        spacing: UM.Theme.getSize("default_margin").width
         anchors.horizontalCenter: parent.horizontalCenter
 
         Button
@@ -34,6 +33,13 @@ Item
             action: Cura.Actions.open
         }
 
+        Item
+        {
+            id: spacing
+            width: UM.Theme.getSize("default_margin").width
+            height: parent.height
+        }
+
         Cura.MachineAndConfigurationSelector
         {
         }