Added a "pcbignore" file concept, which may be used to specify the subdirectories and files to be ignored when constructing the menus of newlib parts in the Library window. Falls back to previous behavior if no pcbignore files are found. Full documentation is in a new appendix in doc/pcb.texi (and derivatives).

Signed-off-by: Bert Timmerman <bert.timmerman@xs4all.nl>