When looking at markers for a merged thread, the thread shown in the tooltip is always "Merged Thread"

STR:

1. Open [this profile](https://profiler.firefox.com/public/36d6d200bb8ec4eff5a90044dc066063703edf6a/marker-chart/?globalTrackOrder=4-5-0-1-2-3&localTrackOrderByPid=39199-0~39202-0~39200-0~39201-0~&thread=3&v=5)
1. Shift click some threads.
1. Hover some markers

=> notice that the displayed thread is always "Merged Thread". Instead we should show the originating thread.

Also the `Thread` line should probably come first rather than last.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-405)