Don't try to use gtk when it is not initialized.
Eliminate pixbuf from IdCard

IdCard should not depend on GDK.
Instead, add utility function to find the correct pixbuf given an IdCard.
For now, it simply returns the "avatar-default" icon, but should eventually
do something intelligent based on the IdCard data.
This incidentally fixes the bug that id's from the keyring data store didn't
get the avatar-default icon.