Changes

Summary

1. more robust: "gtar" is default name in Homebrew;
2. tuned;
3. more direct File.executable operation: avoid external process (on Unix);
4. clarified absolute isabelle_home and (implicitly) isabelle_home_user;
5. tuned messages;
6. more direct File.link operation: avoid external process;
7. eliminated old makedist_bundle and remote_dmg: build_release does everything in Scala; more robust components and classpath via Other_Isabelle; updated macos_app to include full dmg template; misc tuning and clarification;
8. tuned;