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.
d69f7fd63e