Move guitranslations out of i18n and into horizons.gui

The idea is to make i18n as small as possible, it should only find
languages and switch between them and provide a translation interface
for other parts of the code.
Update object translations when LanguageChanged message arrives

I'd like to move this out of i18n completely, but I don't know where to
put it yet.
Merge i18n.utils into i18n