Changes

Summary

1. updated to jedit_build-20181203 (according to d70767e508d7);
2. Components.download similar to "isabelle components", but without unpacking;
3. clarified signature;
4. clarified signature;
5. more explicit support for Isabelle system components; activate_bundled_components: check component dir as in makedist_bundle;
6. more operations;
7. static type for Library.using: avoid Java 11 warnings on "Illegal reflective access"; more uses of "using";
8. tuned;