Mark models that are outside the buildarea in the objectlist

resources/qml/ObjectItemButton.qml

@@ -35,6 +35,7 @@ Button
             text: objectItemButton.text
             font: UM.Theme.getFont("default")
             color: UM.Theme.getColor("text_scene")
+            opacity: (outsideBuildArea) ? 0.5 : 1.0
             visible: text != ""
             renderType: Text.NativeRendering
             verticalAlignment: Text.AlignVCenter

resources/qml/ObjectSelector.qml

@@ -120,6 +120,7 @@ Item
                 }
                 text: model.name
                 width: listView.width
+                property bool outsideBuildArea: model.outside_build_area
             }
         }
     }