Tooltips for menus and buttons

It's a common thing to hover over a button in nemo or firefox and a little tooltip will appear describing what the button does. button% doesn't provide this interface and the help-string from menu-item% doesn't to, uh, do much of anything that I can see. If there's a way to do this already, I'd love to hear, otherwise...

Feature request! Tooltip functionality for menu-item% and button% and, like, other things that might plausibly be hovered over.