Create notion of Set Default Server action to be used for VDB execution and Preview

Because ability to have multiple servers in Designer for 7.0 AND the requirement to utilize VDB's for both preview and execution AND the fact that VDB's require a "Server" to be deployed and queried from, its become apparent that we need a notion of a "Default" server from which these actions are performed.

1) Create a "Set Default Server" action
2) Persist this server (can be null) with the Server Manager
3) Decorate this "Server" in Connectors View differently than other servers