Support eio runtime events #14
                
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On top of https://github.com/TheLortex/eio/tree/runtime-events (which is a rebase of https://github.com/patricoferris/eio/tree/runtime-events)
When eio emits events using the custom events API, mirage-trace-viewer will be able to consume them using this PR.
The PR is incomplete, a command line option is needed to switch between the two formats (ctf / runtime events) (or a separate binary), both for the js and gtk versions.
I have also added support for the GC runtime events, now GC phases are also displayed: