Spreadsheet: struct Cell => class Cell

Hide private members, and make the odd update() -> sheet->update(cell)
-> update(Badge<Sheet>) -> update_data() less odd by removing the
update(Badge<Sheet>) step.

Applications/Spreadsheet/main.cpp

@@ -181,7 +181,7 @@ int main(int argc, char* argv[])
             if (!first)
                 text_builder.append('\t');
             if (cell_data)
-                text_builder.append(cell_data->data);
+                text_builder.append(cell_data->data());
             first = false;
         }
         HashMap<String, String> metadata;