Changes

Summary

1. prefer "Isabelle DejaVu Sans", even for headless batch-build (session_graph.pdf);
2. clarified signature: fonts are not dependent on GUI;
3. tuned signature;
4. clarified signature;
5. avoid loading of font file, to eliminate "Illegal reflective access by com.lowagie.text.pdf.MappedRandomAccessFile$1 (iText-2.1.5.jar) to method java.nio.DirectByteBuffer.cleaner()" -- due to com.lowagie.text.pdf.TrueTypeFont.process() / RandomAccessFileOrArray;
6. proper font file name for HTTP (amending dc9a39c3f75d); clarified Entry content;
7. clarified order;
8. tuned whitespace;
9. clarified symbol groups;
10. more explicit Isabelle_Fonts.Entry; more robust font embedding into PDF and HTML;
11. prefer Isabelle_Fonts.sans for GUI;
12. prefer Isabelle_Fonts.sans (not mono) as derived GUI font;
13. clarified signature;
14. proper file names;
15. clarified modules;
16. clarified Admin resources; tuned comments;