This app - the slideshow - provides a dynamically generated navigation panel.
$::app::slides is an ordered list of slide records: {filename
slide_title}
On loading, the app determines the current slide.
The ::app::nav proc determines the previous, next and parent slide and renders
the panel with tpl/adp/nav.adp
Sub-slideshows are defined in app/adp/*.adp, they override the parent link and
::app::slides. See e.g. the setup slideshow.