Add slave owner to character window

Add slave owner to character window for slaves.

In the latest versions of CSR, slaves can now be owned also by by non-ruler characters (pool characters that are slave traders). For such slaves it is no longer possible to figure out who their owner is in the character UI, so I added an "Owner" portrait in the relations tab. It occupies the slot of the "Diarch" portrait, and since slaves can't have diarchs, there is no conflict.

I am playing with this change since about 2 months now and I think it's free of any issues.