List available socket bindings when creating a JGroups stack

When adding a JGroups stack, the validity of its socket binding is only checked after the whole stack has been defined. In case of a typo, this will lead to the user having to reenter the whole stack configuration, which is usually not trivial.

I propose we either check the validity of socket binding right after the first "Next" button is clicked, and ask the user if he wants to create a new socket binding, if there's no socket binding with the corresponding name.

Alternatively, the socket binding could be selected from a list of available socket bindings.