Show progress during decode

Given that decoding can take quite a while on large traces, it might be a nice UX improvement to show a progress bar over how many events have been processed. This should be possible as long as we can get the number of events in the perf.data file before processing them.

The issue names no files, tests, or entry points. Start by tracing how perf.data events are decoded and how the UI represents long-running work; then determine how to obtain the total event count before processing. Done means large-trace decoding shows progress based on processed versus total events.

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