Let scripts change socket.is_multi_input after init

At the moment we can only set the is_multi_input property on sockets when creating them (via inputs.new() ). However for a custom group node I'm working on, it would also be very useful to be able to toggle that after the group (and it's sockets) have been created. That way sockets can get the "long pill" shape when multi is enabled.

I know I can set link_limit to zero, but it doesn't change the socket shape.