[0.3.1] 16-04-2021

[0.3.0] 15-04-2021

[ExpandNodeIcon] and [NodeWidgetLeadingIcon] are able to expand/collapse leaf nodes.
[TreeNode.children] changed from [List] to [Set] to avoid duplicate children.
The [TreeViewController] received a new callback.
- The new onAboutToExpand is useful to dynamically populate the [TreeView].
Some performance improvements when expanding/collapsing nodes.

Added useBinarySearch option to TreeViewController.
Many performance improvements by reducing the amount of loops that expandNode and collapseNode took.

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.