Add GUI password dialog / API

[GoogleCodeExporter]

Add a dialog box / API for opening the file.

API should be along the lines of:
{{{
public class PasswordDialog
{
    public static OfficeCryptoStream OpenStream(String file)
}
}}}

Try to open the steam as plaintext first. If unsuccessful, show the password 
dialog until success or Cancel of the operation (returning null).

Original issue reported on code.google.com by [email protected] on 7 Oct 2009 at 8:56