Add '+' and '-' buttons to the view-based Source List example.
Improve SourceListItem API for adding/removing children.

Back the `children` property with a mutable array and implement -addChildItem:
and -removeChildItem: as convenience methods for adding and removing children.
Implement an -addButtonAction: method for the view-based example.
Allow the label for items not in the "Library" group to be edited.
Implement the remove button action method in the view-based example.