more tight HTML output: avoid extra lines within <pre>;
avoid spurious output after exit;
proper dynamic controls, notably for auto_update_enabled;
tuned signature;
HTML GUI actions via JavaScript;
clarified defaults;
GUI controls similar to Tools/jEdit/src/state_dockable.scala;
tuned;
clarified modules; --HG-- rename : src/Tools/VSCode/src/preview.scala => src/Tools/VSCode/src/preview_panel.scala rename : src/Tools/VSCode/src/state.scala => src/Tools/VSCode/src/state_panel.scala