Merge branch 'labeling-improvements' into pre_release_tests_nov2020

com.unity.perception/Editor/GroundTruth/LabelConfigEditor.cs

 
             m_AddNewLabelButton.clicked += () => { AddNewLabel(m_AddedLabels); };
 
+#if UNITY_2020_1_OR_NEWER
+            m_LabelListView.onSelectionChange +=  UpdateMoveButtonState;
+#else
-
+#endif
             m_RemoveAllButton.clicked += () =>
             {
                 m_SerializedLabelsArray.ClearArray();