Secure GUI Programs

I've been thinking about writing graphical utilities (e.g. a file manager), but I'm not sure whether I should be using Qt. Can I consider Qt secure for things such as rendering filenames from untrusted filesystem servers? If not, what do you suggest?

Also, is there a file manager anyone has already started making?

Another question: for previews/thumbnails (e.g. for icons), is it reasonably efficient to render the preview for each file in its own process, or should I batch them together?