Make re-labelled trace names persistent

At the moment #451 allows labelling a trace set with a user defined name (without impacting the file name). However, this label isn't persistent since the change is just made in the front end and is not propagated to the trace server. For example, if a user closes the trace viewer, when they reopen it the traces will have their original filenames instead of the relabeled names. This is unexpected and confusing.

Trace set labels should be propagated to the server and stored so that they are persistent.

Start by tracing the frontend relabeling behavior introduced by #451 and how trace-set data is sent through the Trace Server Protocol. Confirm where the server stores and returns trace set labels, then verify that relabeled names remain after closing and reopening the trace viewer.

Written by the indexing model from the issue text.