Define a new stateless widget called ActionButton to display
these round buttons.

Pass a few instances of this new ActionButton widget into your
ExpandableFab.

Expand and collapse the action buttons

The child ActionButtons should fly out from under the open
FAB when expanded. Then, the child ActionButtons should
fly back under the open FAB when collapsed.
This motion requires explicit (x,y) positioning of each
ActionButton and an Animation to choreograph changes to
those (x,y) positions over time.

Introduce an AnimationController and an Animation to
control the rate at which the various ActionButtons expand and collapse.

Next, introduce a new stateless widget called _ExpandingActionButton,
and configure this widget to animate and position an individual ActionButton. The ActionButton is provided as a generic Widget called child.

Finally, use the new _ExpandingActionButton widget
within the ExpandableFab to complete the exercise.