Mark rows and add a warning that changes won't be saved until the form is submitted

tabledrag has a nice feature where if you re-order a row, the row gets marked with a yellow background and there's a warning generated at the top of the table that says:

* Changes made in this table will not be saved until the form is submitted.

It might be a good idea to do something similar for other operations on the table. For example, newly uploaded files could get the same treatment (to remind people that pressing "upload" isn't enough) and perhaps also marking files hidden/shown, etc.

Not sure how easy it is to reuse tabledrag's functionality here. I know we're already instantiating a tabledrag JS object in our custom JS, so perhaps it's quite easy.