ui/ncurses: Use a separate type for help text

Because it's initialised statically, help text won't be directly
gettext()-ed. Instead, we need to perform the gettext translation at
runtime, and pass untranslated strings into the help_screen code.

Instead of trusting callers to pass the untranslated strings though, we
encapsulate the help text data into struct help_text, so we know we have
an unstranslated string.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>