RFR: JDK-8262992: Improve `@see` output

[Hannes Wallnöfer]

This changes the output for `@see` tags to a `<ul>` structure. A different CSS style is used if any of the `@see` tag labels are longer than 30 characters or contain a comma. 

The layout for the default CSS style is similar to the one we had so far with multiple links displayed inline and separated by commas. In the new code, the commas are not hardcoded but generated via CSS. The CSS style for lists containing longer labels displays the list in block mode, but still separated by commas and without list markers. Note that the commas are generated via CSS in both cases.

As expected, there's a lot of test churn with this change. I converted some tests to text blocks that were still using old style string concatenation. In `TestSingletonLists.java` I had to exclude `@see` lists from the singleton check as javadoc generates a "See also" item for constant field values.

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Commit messages:
 - JDK-8262992: Improve `@see` output

Changes: https://git.openjdk.java.net/jdk/pull/2894/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2894&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262992
  Stats: 396 lines in 20 files changed: 176 ins; 20 del; 200 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2894.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2894/head:pull/2894

PR: https://git.openjdk.java.net/jdk/pull/2894