Add visible marker for modified read-only buffer.

Hi, I really like `smart-mode-line`, I've been trying it out for a few days and it's definitely something I'm gonna keep.

There's one small issue, however, that I'm hoping to address with my patch. Emacs allows for a read-only buffer to be modified and the standard mode line shows this as `%*`. I use this possibility in a package of mine (Ebib), but unfortunately, `smart-mode-line` doesn't represent this status, it just shows the buffer as read-only.

With this patch, `sml/read-only-char` is displayed with `sml/modified` face instead of `sml/read-only-face` if a read-only buffer is modified. The patch switches the `buffer-modified-p` and `buffer-read-only` clauses in the `cond` in `sml/generate-modified-status` and makes the character used in the `buffer-modified-p` clause dependent on the read-only status of the buffer.

I did it this way instead of the other way around (i.e., making the face in the `buffer-read-only` clause dependent on the modified status) because a modified read-only buffer can be saved if it’s visiting a file, so I figured it should have the help text and mouse-1 binding of a normal modified buffer.

Would be great if this patch or something like it could be included. Thanks!