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!