Frontend i18n using PHP gettext module

As discussed, a gettext support should be added to frontend.

1. All strings that present in current locale files should be embed for gettext translation;
2. po/mo dictionaries should be generated;
3. All new strings should be put directly into code (not in includes) embed as gettext functions;
4. Zabbix gets gettext PHP module dependency, so a check in the wizard should be added.