show_page runs a page's close/open actions, run_timers fires due timers (at most 16 a call),
next_timer and script_clock drive the viewer. Fields they show, hide or give another icon
are drawn so on screen only (screen_config); values and resets are undoable edits. A timer's
alerts and requests go to the console. getOCGs layer changes turn layers as the panel does.
Co-authored-by: Peter Yatsyshin <p.yatsyshin@googlemail.com>