Allow id's to be used in tables in Admin CP

I was working on a small feature (basically checking if an idea was possible), and noticed that when calling `$table->construct_cell()` with an argument for $extra listing `id` as an attribute was not displaying properly.

For example, calling `$table->construct_cell("foobar!", array("class" => "align_center", "id" => "foo"))` would result in  `<td class="align_center" style=""general>foobar!</td>`. This is because the `$cell['extra']['id']` was being concatenated onto the cell directly instead of an attribute being set, and this commit fixes it. It's a very minor commit but it was an annoying error to debug. Now the above code will result in `<td class="align_center" style=""  id="general">foobar!</td>`