Add a tab to the GitTimingsView that renders a box plot of the observed durations of a specific operation over time. To begin with, let's focus on the staging and unstaging actions. This is a soft prerequisite to doing further performance work because it'll allow us to measure how far we are from our performance targets and to know when we've reliably hit them.

Fixes #616.

Remaining Work

• Refactor the GitTimingsView to accommodate multiple tabs.
• Implement the BoxPlot tab.
  • Add the BoxPlot component as a tab of the GitTimingsView.
  • Compute the minimum, quartiles, and maximum of a data series.
  • Render each data series as a box with a label.
  • Display raw data series characteristics within a tooltip.
• Collect performance data with the EventHandler
  • Refactor the EventHandler to invoke callbacks synchronously as well as resolve Promises on events
  • Rename the EventHandler to Switchboard
  • Pass a Switchboard down through the component hierarchy
  • Observe the initiation and completion of staging and unstaging events
  • Subscribe to Switchboard events within the GitTimingsView to collect performance data
• ...