Add Stack to Notes Pane to show File and Project Navigation

[kmwallio]

Add an option for showing a Navigation Hierarchy where the notes are. It should allow alternating between Notes / File Navigation / Project Navigation.

Clicking on a heading should switch to the file with the heading in view.

On Markdown, the navigation should just show # Headings. Fountain should show # Headings and INT./EXT. Scene Headings.