In checkbox mode, on right click add the item to the selection instea…

…d of clearing the selection

Closes #485

source/BulkCrapUninstaller/Forms/Windows/MainWindow.cs

@@ -1226,7 +1226,7 @@ private void uninstallerObjectListView_CellRightClick(object sender, CellRightCl
 
             if (uninstallerObjectListView.CheckBoxes && !uninstallerObjectListView.IsChecked(e.Model))
             {
-                uninstallerObjectListView.UncheckAll();
+                //uninstallerObjectListView.UncheckAll();
                 uninstallerObjectListView.CheckObject(e.Model);
             }
         }