Remove setters from DocumentModel
Remove setters from FolderModel
Remove setter from SymbolLink
Remove setter from UrlLink
Remove setters from Token.

I'm not sure if I'm doing this right, but I've also added methods that
allow us to create new tokens with trivia and links.
Remove setters from Trivia
Remove setters from WorkspaceModel
Ensure we persist the "IsDeclaration" info.
Merge branch 'ImmutableModel' of https://github.com/JoshVarty/SourceBrowser into JoshVarty-ImmutableModel
Merge branch 'JoshVarty-ImmutableModel'
Fixed merge conflict