shouldRefresh method
Checks if the TreeNode with id needs to be refreshed (update lines, ...).
A TreeNode that is marked as needing refresh will only rebuild itself, to rebuild an entire subtree, use refreshNode.
This is currently only used to update the lines of a node when it's sibling list changes. (If nodes are kept expanded when a new sibling is added, the lines are not updated and the new node lines are not connected.)
Implementation
bool shouldRefresh(String id) => _nodesThatShouldRefresh[id] ?? false;