Make it possible to provide a renderable as a label to a progress bar

Other components do it (`<TabbedArea>` for the tab label comes to mind), I don't see why we wouldn't want to generalize a bit here, if anyone wants to add a child to an already nested progress bar or if someone wants better control over how the percentage is displayed (currently the value is ceiled).
I thought about it, but not with this approach! That makes sense.