Allow a subject to swear secret fealty to multiple empires

[Tamwin5]

Currently, a subject can only swear secret fealty to a single empire. This has several effects:

1. It's really easy in MP to tell if any of your subjects are swearing secret fealty. Just ask a friend to see if they can ask for secret fealty (since the option is grayed out).
2. It's impossible to get an AI to change secret fealty. Even if they would say yes to you, if they are already swearing secret fealty there is nothing you can do. Other then just kill/vassalize the target of the secret fealty, but you don't know who that is.

Now, the potential for multiple simulatanious secret fealty wars would be a problem. As such, the declaring of a secret fealty war would automatically end all other secret fealties that the subject had.