Add translation for main menu

Add engine.gettext() and remove gettext() calls in guiFormspecMenu.cpp

doc/menu_lua_api.txt

@@ -167,6 +167,13 @@ engine.file_open_dialog(formname,caption)
 ^ returns nil or selected file/folder
 
 Helpers:
+engine.formspec_escape(string) -> string
+^ escapes characters [ ] \ , ; that can not be used in formspecs
+engine.gettext(string) -> string
+^ look up the translation of a string in the gettext message catalog
+fgettext(string, ...) -> string
+^ call engine.gettext(string), replace "$1"..."$9" with the given
+^ extra arguments, call engine.formspec_escape and return the result
 dump(obj, dumped={})
 ^ Return object serialized as a string
 string:split(separator)