[tools] callstats improvements

- Sort by labels if no category is selected
- Support page separators in simple txt files.
- Fix un-toggling subcategories
- Prompt for labels when loading multiple files
- Improve parsing performance by using raw for-loops and fixing regexps

Change-Id: Ibd388e4134b4c0722a7f44d7eb4c5c56748e5175
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2759511
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Patrick Thier <pthier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73459}