More broadly, this just supports multiple profiles loaded into the editor in the same time, which supports import from profiles which are multithreaded by importing each thread as a different profile.
For now, the only two file formats that support multiprocess import are Instruments .trace files and speedscope's own file format
In the process of doing this, I refactored the container code considerably and extracted all the dispatch calls into containers rather than them being part of the non-container view code. This is nice because it means that views don't have to be aware of which Flamechart they are or which profile index is being operated upon.
Fixes#66Fixes#82Fixes#91
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