This change adds an actual background worker for listening to project
change notifications and starts sending updates when the project's razor
dependencies change.
I had to do a litle surgery to get things working. There were plenty of
small bug fixes.
Additionally I got rid of the WeakReferences for tracking listeners. I
was seeing TextBuffers hanging around in VS longer than I expected and
the WeakReferences weren't getting cleaned up. I think it's better that
we just track the lifetime.
6e6a24cbb4