A debug label for this widget.
Not used for anything except to be printed in the diagnostic output from
toString or toStringDeep. Also unused if a focusNode is provided,
since that node can have its own FocusNode.debugLabel.
To get a string with the entire tree, call debugDescribeFocusTree. To
print it to the console call debugDumpFocusTree.
Defaults to null.
final String debugLabel