An expandable app bar.
The expandable app bar allow you to have a hidden content which can be shown
if the user drags the bar up.
barButtons allows to pass a widget (typically a row) that contains the buttons
which will handle the app bar functionality.
child widget that contains the hidden content.
scaffoldChild widget that contains the hidden content, but this hidden content
is wrapped inside a Scaffold widget. Only child or scaffoldChild is allow no
both at same time.
opacity the oppacity of hidden content.
stopOnDrag if true when the user stops dragging the hidden content will stay visible
if false the hidden content will be expanded/narrowed to the top or the end of the screen and then disappear.
autoHide hides the app bottom bar while the user is dragging the hidden content. [...]