Enhancement: Add Leo-Editor-like outlines/clone nodes/views

[Joshua Olson]

I'd like to be able to specify and see live-updated parts/views of other documents in, for example, section X of TODO_TODAY.txt. So the text in each view (node in Leo parlance) would either be duplicated each place it's needed, or just referenced with a goto search command if all users of the document with the cloned view expect to open that document with zed.

Basically, this.