Improve goto decl

Not for merge, just for discussion and help. 
#556

In this PR, navigating to an external type (`System.String`) works. You are taken to the correct type in object browser, without error message. But I can't get members to work properly (`System.String.Copy`) 

Anyone have a clue why it doesn't work? I'm not sure what NRefactory wants here to be able to find the member. Please check the diff and help!
No, just that we were using the wrong overload. My guess is we had something like (nsp, lastIdent ...) in this constructor previously. This would be the correct overload. Then we started naming params, and a different overload was chosen when we added `Name=`.