Add small activity graphs to the dropdown that lists all threads

Sometimes I have the question "Which threads are doing work, and when?"
If some tracks are hidden, either because they were hidden automatically or because I hid them myself, it becomes a bit cumbersome to get that information: I have to make the threads visible, then look at them in the track list, and then hide them again if they turn out to be not interesting. This becomes annoying if there are many threads.

It would be a lot nicer to get an overview of whether threads are busy without having to display them in the main view. Having activity graphs for all threads in the dropdown would provide this information.

(This was originally part of my dropdown proposal in https://github.com/firefox-devtools/profiler/issues/1410#issuecomment-433076718).

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-181)