Stable null safety release.

Refactoring of the entire API:

• The old API was very "janky" with AnimatedList, TreeView now uses ListView.custom under the hood.

• New InheritedTreeView inherited widget is used under the hood to get access to TreeViewController and TreeViewTheme from TreeView.of() method.

• New ScopedTreeNode inherited widget, to get a TreeNode from anywhere under it in the widget tree, every TreeNode has its own ScopedTreeNode.

• New NodeWidgetLeadingIcon for a folder/file icon approach.

• The TreeViewController is now a ChangeNotifier, to talk to TreeView more easily.

• TreeNode received a delete() method that removes it from the tree and clear its relationships.

Extracted the main implementation of TreeViewController to a base class to be able to change the expansion of a node without notifying listeners.

• Added some tests to TreeViewControllerBase.

Updated NodeWidget to a Row instead of a ListTile

• ListTile had some errors when using LinesWidget as leading.

Dropped support for selecting/disabling nodes as it was getting very complicated and "hacky" to manage.

Dropped TreeLine.link as it was complicating the indentation of nodes.

Plus Many upgrades and fixes.