New TreeView API

• 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.

First release.

Rename package to flutter_fancy_tree_view

The old name was very similar to other packages
pub rejected the publish.