resources/forms/preferences.ui: add full licenses and small fixes

The OpenSSL license is taken from the Debian openssl_1.0.1c-4 package,
file /usr/share/doc/openssl/copyright.  The zlib license from the
zlib1g_1.2.7.dfsg-13 package, file /usr/share/doc/zlib1g/copyright.
The Xpdf license from the Debian xpdf_3.03-10 package, file
/usr/share/doc/xpdf/copyright, with the full GPL-2 version from the
Debian base-files_6.12 package, file /usr/share/common-licenses/GPL-2,
used for the QuaZIP license as well.  The Open-Sankore license is the
COPYING file from commit 4b8d3b74.