manage folding via sidekick by default;

eliminated obsolete heading category -- superseded by heading_level;

treat main theory commands like headings, and nest anything else inside;

proper treatment of equal heading level;

added missing Keyword.THY_SCHEMATIC_GOAL; more keyword categories;

default Sidekick parser based on section headings;

some support for nested source structure, based on section headings;

tuned;

misc tuning and simplification, using Isabelle_Sidekick.Asset (not sidekick.Asset, which is dynamically dispatched to slightly different semantics);

updated version;

private counter, to keep externalized ids a bit smaller;

added general Synchronized.counter convenience;

explicitly identify forked/joined tasks;

accomodate old manuals that include pdfsetup.sty without isabelle.sty;

merged