Fix TreeView selection state for unexpanded items

[klondikedragon]

Resolves #578

Pre-launch Checklist

• I have updated CHANGELOG.md with my changes
• I have run "dart format ." on the project
• I have added/updated relevant documentation

[bdlukaa]

Why don't we build the children here?

[klondikedragon]

The build method only returns visible children, and the selection state also needs to consider children that are not visible because they are underneath a non-expanded item. Also, updateSelected is being called on all parent nodes for any TreeViewItem whose selection state has changed in a depth-first fashion (or in the case of the initial load, for all items). If updateSelected recursively builds all children again, it will make the traversal cost of updating selection state O(n^2) in the size of the tree rather than O(n). This could be a big difference for really large trees.

I've added some more documentation to updateSelected to indicate the proper way to call it, for future references. All current callers already do follow this pattern.

item.executeForAllParents((parent) => parent
  ?.updateSelected(widget.deselectParentWhenChildrenDeselected))