Complete 2-qubit gate player abilities

[WrathfulSpatula]

The Eigenpong interface already enact single qubit gates on the Eigenpong ball when the UI gates are active. We need 2-qubit gates as well.

I was envisioning a system where, when a player activates a 2-qubit gate on one ball, they have the option of creating a new ball that acts as target qubit for the 2-qubit gate, with control qubit being a ball bounced off the player's paddle with a 2-qubit gate active.

[WrathfulSpatula]

(This needs a user story.)

[WrathfulSpatula]

"As a player, I want to enact a 2-qubit gate, so that two Eigenpong balls become entangled."