contact widget: Use dim label for Tox ID

When a contact doesn't have a name, and we use their Tox ID as a name
replacement, use the 'dim-label' style class to indicate that it's not
their actual nick.
contact widget: Icon for never seen contacts

When we have never seen a contact yet, use the 'user-invisible' icon
instead of the usual offline icon. This uses wrong semantics but I don't
know a better icon yet.
welcome widget: Show our own Tox ID