Idea: Hide or mute idling threads


_Only one thread actually has data, the rest is empty_

_User story:_ When slicing a profile down to a time range, I want to be able to tell quickly which threads are actually active, so that I can focus on the threads that have data.

It feels like the empty white (no data) or gray (not started) thread timelines just take up precious space. We could mute the empty ones or hide their timelines. Hiding the whole track might be too noisy when think about sliding the range left and right while tracks pop in and out.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-274)