Create general.dtd

This is missing Czech translation.

BTW - 
1) I inserted into my sogo-connector-24.0.0.xpi (and I installed it again into TB), but it did'nt show in Thunderbird. So it looks that TB/Connector didn't use this file. 
2) TB/Connector don't use file chrome/locale/cs-CZ/sogo-connector/preferences/general.dtd too.

Milos