IconTheme: Never fail a lookup or icon load

IconTheme: Never fail a lookup or icon load

If icon lookup fails or if loading it fails later, just always
fall back to the built in image-missing icon. Nobody is handling
missing icons in a sane way anyway.

If you *truly* need to handle missing icons, you need to manually
use gtk_icon_theme_has_icon().

While changing the loading code I also fixed an issue where it
was always passing "png" to pixbuf, now it also handles "xpm" if
that is the filename suffix.