e-d-s: load icon regardless of the mime type being available

Also, allow Edsf.MemoryIcon to be instantiated with a null
image type. The notion of the image type not being available
was half there (some comments mentioned the possibility), but
it wasn't contemplated in the constructor and in the instance
variable.

Fixes: https://bugzilla.gnome.org/show_bug.cgi?id=662770