Manage option translations in separate .po files to make translations

Manage option translations in separate .po files to make translations
easier for non-programmers familiar with GNU gettext. For now there is
only a German translation but more translations are encouraged.

Translatable strings are extracted from t_options.h containing option
definitions with English descriptions. A corresponding header options.h
with all available translations of option descriptions is generated by
gen_xmlpool.py and included by xmlpool.h.

Many option descriptions were rephrased to make them unambiguous for
translation without context. They are hopefully more helpful to users at
the same time.