eliminate MuttIndexWindow from mutt_make_string_flags()

eliminate MuttIndexWindow from mutt_make_string_flags()

`mutt_make_string_flags()`, and its macro wrapper `mutt_make_string()`,
use `mutt_expando_format()` and `index_format_str()` to expand strings.

Some of the callers expect the string to be truncated at the screen
width.  These functions now have an extra paramter to avoid using
`MuttIndexWindow` directly.