tuned imports -- accommodate scala-2.13.0-RC3;
more general support for Isabelle/PIDE file formats -- less hardwired Bibtex operations;
uniform preview for Isabelle/jEdit and Isabelle/VSCode;
clarified modules;
tuned output;
tuned signature;
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