RFR: 7904082: The UI for API Diff could be improved

[Jan Lahoda]

This PR intends to improve the result of API diff in a few ways:
- in the list of updated elements, using green for elements that were added (instead of elements that are unchanged), and red for removed elements. At least in cases two variants of an API are compared.
- making added/removed elements more prominent by also coloring the signature lines
- hiding unchanged elements by default.

An example diff that is produced before this patch:
https://cr.openjdk.org/~jlahoda/CODETOOLS-7904082/before/
after this patch:
https://cr.openjdk.org/~jlahoda/CODETOOLS-7904082/after/

Please let me know what you think. I'll see if I can reasonably add some tests for this functionality.

-------------

Commit messages:
 - Cleanup.
 - Fixes.
 - Prototype of dynamic hiding of unchanged elements.
 - More prominent coloring of added/removed elements.
 - Changing stylesheet
 - Hide unchanged.

Changes: https://git.openjdk.org/apidiff/pull/32/files
  Webrev: https://webrevs.openjdk.org/?repo=apidiff&pr=32&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7904082
  Stats: 249 lines in 4 files changed: 161 ins; 35 del; 53 mod
  Patch: https://git.openjdk.org/apidiff/pull/32.diff
  Fetch: git fetch https://git.openjdk.org/apidiff.git pull/32/head:pull/32

PR: https://git.openjdk.org/apidiff/pull/32