<i18n dev> RFR: 8347122: Add missing @serial tags to module java.desktop

[Hannes Wallnöfer]

Please review a doc-only change to add missing `@serial` javadoc tags in module `java.desktop`. This is a sub-task of [JDK-8286931] to allow us to re-enable the javadoc `-serialwarn` option in the JDK doc build, which has been disabled since JDK 19.

[JDK-8286931]: https://bugs.openjdk.org/browse/JDK-8286931

For private and package-private serialized fields that already have a doc comment, the main description is converted to a block tag by prepending `@serial` since these fields do not require a main description. For protected and public serialized fields that require a main description, an empty `@serial` block tag is appended to the doc comment instead. The effect is the same, as the main description is used in `serial-form.html` if the `@serial` tag is missing or empty. For those fields that do not have a doc comment, a doc comment with an empty `@serial` tag is added. 

This change has no effect on the generated documentation (apart from muting the warnings for missing `@serial` tags when building documentation with `-serialwarn` enabled).

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Commit messages:
 - 8347122: Add missing @serial tags to module java.desktop

Changes: https://git.openjdk.org/jdk/pull/22979/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22979&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8347122
  Stats: 173 lines in 47 files changed: 39 ins; 0 del; 134 mod
  Patch: https://git.openjdk.org/jdk/pull/22979.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22979/head:pull/22979

PR: https://git.openjdk.org/jdk/pull/22979