Add support for merging traces

[LalitMaganti]

It's become clear that we need some way to have multiple independent traces (collected on either the same device or different devices) visualized and analyzable on a single timeline.

Most of the work for this already exists in the trace processor but it is very hard for anyone who does not understand a lot of the internals of TP to make use of it.

This bug tracks the remaining implementation which involves some small work in TP and then some non-trivial work in the UI to expose this functionality.