We need a way to make the most relevant markers be visible in the Marker Chart without scrolling up and down

We have lots of markers, and seeing them all in the Marker Chart requires scrolling up and down, which isn't practical.

Some markers are generally useful, and some others are useful only for engineers with a specific interest in an area of the code base.

In some cases (eg. markers of the Test category), we can guess that they are the most relevant, because they only exist when profiling an automated test (I made them appear first in #3256 as a temporary measure).

This could also apply to UserTiming markers that are likely to be very relevant.

We need a better long term solution to make the most relevant markers most visible.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-375)