Add group spacing

[GASTDASH]

Add groupSpacing to easy create space between each group.

  GroupedListView<dynamic, String>(
    elements: _elements,
    groupBy: (element) => element['group'],
    groupSeparatorBuilder: (String groupByValue) => Text(groupByValue),
    itemBuilder: (context, dynamic element) => Text(element['name']),
    groupSpacing: 64, // group spacing
  ),