This CL unifies the custom events by creating
classes specialised based on the event type.
Multiple entry selection causes panels to
emit 'showentries' event. Single entry selection
causes panels to emit 'showentrydetail' event.
The events are received by the controller App class
and updates the view of the panels and state of the app.

Bug: v8:10644

Change-Id: Ibe26223459ba605c6d6d3f0025bf3a556dfb0578
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2335188
Commit-Queue: Zeynep Cankara <zcankara@google.com>
Reviewed-by: Sathya Gunasekaran  <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69286}