Rename Norwegian Bokmål po file to nb_NO.po

Norway has two officially recognized languages: Bokmål Norwegian (ISO 639-3 nob)
and Nynorsk Norwegian (ISO 639-3 nno). IETF language tags for them would be
nb-NO and nn-NO, respectively.

Thanks Bjørn Lie <zaitor@opensuse.org> for the patch.

Fix Trac ticket 503
Fix GitHub issue #48
Update po/POTFILES.in
configure.ac: fix typo
configure.ac: sorted and updated ALL_LINGUAS
Added and updated translation files from Transifex
Update headers and copyrights of translation files