Add a "show-bottom-bar" property or something similar for AdwBottomSheet

Currently, there is no clean way to temporarily hide the bottom-bar child of an AdwBottomSheet. There are a few scenarios in which this behavior might be useful: for instance, a music player app might want to have a bottom bar showing at nearly all times, but hide it when no song is currently playing.

There are workarounds to allow you to do this, but, it would be very convenient to have something like an AdwBottomSheet:show-bottom-bar property. The default value could be true, but, when false, the bottom bar would be hidden, and the sheet child would become inaccessible. Ideally, there could also be a show/hide animation upon property's value changing.