Previously, when a VisualStudioWorkspace was constructed we were
doing a bunch of work in the constructor talking to the UI thread. This
was dangerous because we might get composed on a background thread.
This moves the dangerous initialization until we're creating a project
when we know we have a UI thread available.

In this refactoring I moved the project tracker and a few other fields
into a private class so it's more obvious what's dangerous to touch.
I also do a pass through all consumers of the property and either
update it to use the deferred state directly (because we're only
consuming projects, so it's fine if we don't have anything) or to the
factory method if we actually are creating projects.

This does temporarily regress the ability to create projects on
background threads, but it was unclear if that was generally safe to
start with.