Fix Showcase CustomDataGrid sample's cell event handling

The sample didn't add the columns to the DataGrid (they were only
used by the custom CellTableBuilder), so the events weren't sunk
correctly.

Bug: #9176
Change-Id: If27f86158c6b922f0d8380c0a9c4f06b5711bd26
diff --git a/samples/showcase/src/com/google/gwt/sample/showcase/client/content/cell/CwCustomDataGrid.java b/samples/showcase/src/com/google/gwt/sample/showcase/client/content/cell/CwCustomDataGrid.java
index ce2fd69..52d642d 100644
--- a/samples/showcase/src/com/google/gwt/sample/showcase/client/content/cell/CwCustomDataGrid.java
+++ b/samples/showcase/src/com/google/gwt/sample/showcase/client/content/cell/CwCustomDataGrid.java
@@ -661,6 +661,7 @@
         return dataGrid.getSelectionModel().isSelected(object);
       }
     };
+    dataGrid.addColumn(checkboxColumn);
     dataGrid.setColumnWidth(0, 40, Unit.PX);
 
     // View friends.
@@ -696,6 +697,7 @@
         dataGrid.redrawRow(index);
       }
     });
+    dataGrid.addColumn(viewFriendsColumn);
     dataGrid.setColumnWidth(1, 10, Unit.EM);
 
     // First name.
@@ -720,6 +722,7 @@
         ContactDatabase.get().refreshDisplays();
       }
     });
+    dataGrid.addColumn(firstNameColumn);
     dataGrid.setColumnWidth(2, 20, Unit.PCT);
 
     // Last name.
@@ -744,6 +747,7 @@
         ContactDatabase.get().refreshDisplays();
       }
     });
+    dataGrid.addColumn(lastNameColumn);
     dataGrid.setColumnWidth(3, 20, Unit.PCT);
 
     // Age.
@@ -760,6 +764,7 @@
         return o1.getAge() - o2.getAge();
       }
     });
+    dataGrid.addColumn(ageColumn);
     dataGrid.setColumnWidth(4, 7, Unit.EM);
 
     // Category.
@@ -786,6 +791,7 @@
         ContactDatabase.get().refreshDisplays();
       }
     });
+    dataGrid.addColumn(categoryColumn);
     dataGrid.setColumnWidth(5, 130, Unit.PX);
 
     // Address.
@@ -802,6 +808,7 @@
         return o1.getAddress().compareTo(o2.getAddress());
       }
     });
+    dataGrid.addColumn(addressColumn);
     dataGrid.setColumnWidth(6, 60, Unit.PCT);
   }
 }