Make it possible to prevent teleporting between VST instances

[mgunyho]

After thinking about #5 a bit more, it might be a good idea to scope the teleport locally within a VST plugin instance by default, and have a right-click option to change to "global mode".

I don't know exactly how to do this, maybe change the keys of the static std::map<...> inputs to (string, InstanceID) or something, where InstanceID would be null for the "global" option. Should check the Rack API to see if there's a way to get some sort of plugin instance ID.