Prepare the system analyzer to be able to select events related to a
a single code log entry.

- Rename source-panel to script-script panel
- Update main index.css to support selects in the panel selection
  header

Bug: v8:10644
Change-Id: Ie8dd1839294687cb9e25995bcb7ef246a7d7f48d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2604707Reviewed-by: Sathya Gunasekaran  <gsathya@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71948}