This adds an option to allow callers to specify that computing
the children of the hierarchy should be delayed to when the
user calls for them, by utilizing the tree-widget :expander
property.
* lisp/emacs-lisp/hierarchy.el (hierarchy-add-tree)
(hierarchy-add-trees): Add parameter 'delay-children-p'.
* lisp/emacs-lisp/hierarchy.el
(hierarchy--create-delayed-tree-widget): Add function.
* lisp/emacs-lisp/hierarchy.el (hierarchy-convert-to-tree-widget):
Utilize ':expander' if delaying children. (Bug#55900)
* test/lisp/emacs-lisp/hierarchy-tests.el: Add tests for
delayed-children functionality.
