Allowing combine to work on a single item if it has multiple meshes.

MatterControlLib/ApplicationView/ApplicationController.cs

@@ -683,7 +683,11 @@ namespace MatterHackers.MatterControl
 					TitleResolver = () => "Combine".Localize(),
 					Action = (sceneContext) => new CombineObject3D().WrapSelectedItemAndSelect(sceneContext.Scene),
 					Icon = AggContext.StaticData.LoadIcon("combine.png").SetPreMultiply(),
-					IsEnabled = (scene) => scene.SelectedItem is SelectionGroupObject3D,
+					IsEnabled = (scene) =>
+					{
+						var selectedItem = scene.SelectedItem;
+						return selectedItem != null && selectedItem.VisibleMeshes().Count() > 1;
+					},
 				},
 				new SceneSelectionOperation()
 				{