Add option of having a target text in "Go to implementation" / find implementations responses

There currently exists no way to receive the "target text" for a
findimplementations / GotoImplementationRequest. This is inconsistent,
given that a findusages / FindUsagesRequest provides them.

FindUsagesResponse "target texts" are currently provided using the
QuickFix class, whereas the GotoImplementationResponse uses a Location
class (which does not have a "Text" member).

This change provides a way to get a listing of QuickFix responses for
a GotoImplementationRequest. It should break no existing code, since
this feature is provided as a new API called
"findimplementationsasquickfixes".