Select a new document when trashing a multiple selection

(Not doing so resulted in either a trashed item being selected when
returning to board mode, or a crash if the item had been permanently
deleted)

src/document/UBDocumentController.cpp

@@ -857,7 +857,7 @@ void UBDocumentController::deleteSelectedItem()
             deleteTreeItem(item, false, false);
         }
 
-        //selectADocumentOnMultipleTrashing();
+        selectADocumentOnMultipleTrashing();
     }
 
     else if (mSelectionType == Document || mSelectionType == Folder) {