Consider adding a row to the end of the timeline that says "+ N more hidden tracks" to make hidden tracks more explicit

Right now, it's not really discoverable to the new users that we have hidden tracks and they should look and find a track when they are looking for a process/thread. And people usually go to the bottom of the timeline to search for something.

To make this more explicit, we can add a row to the end of the tracks in the timeline that says "+ N more hidden tracks". That way, when they search for something and go to the bottom, they can see that there are actually some tracks that are hidden. Also clicking on that item would open the track context menu.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-460)