MimeTypes: Refresh exe icon, add missing symlink

Updates the `exe` icon to look more rounded like other executables.
Colors depart from upstream slightly as the darker upstream color with
darker emblem is somewhat difficult to see. This one uses a stronger
contrast between the two.

Add a `portable-executable` symlink as some `exe` files were not using
the correct icon. Now all files with the `exe` extension will get the
same icon.

This has the unfortunate side effect that `dll` files will also
use this icon, but that's an issue with the FreeDesktop.org
shared-mime-info database. Until they're given unique entries
in the database (if possible), they'll have to share an icon.

Author: newhoa

elementary-xfce/mimetypes/128/application-vnd.microsoft.portable-executable.svg

@@ -0,0 +1 @@
+application-x-msdownload.svg