Changes

Summary

1. clarified package name (actually both pxfonts and txfonts exist and have this font);
2. tuned;
3. prefer isabelle bbbfont;
4. enforce full build;
5. clarified symbol names, notably relevant for Z_Notation;
6. update README (actually after update of component);
7. high-quality blackboard-bold fonts from "txmia" (package "txfonts");