It's non-obvious when speedscope imports a multi-threaded profile

When profiles are imported with multiple threads, those threads are accessible via the top bar. This is so non-obvious that even though I made this feature, when I was debugging a performance issue where I expected the profile to be single threaded, I thought I was using the profiler wrong.

There should be some kind of indication that the imported profile is multi-threaded in a way that draws more attention to itself than a grey `(1/2)`.