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;

proper font file name for HTTP (amending dc9a39c3f75d); clarified Entry content;

clarified order;

tuned whitespace;

clarified symbol groups;

more explicit Isabelle_Fonts.Entry; more robust font embedding into PDF and HTML;

prefer Isabelle_Fonts.sans for GUI;

prefer Isabelle_Fonts.sans (not mono) as derived GUI font;