RFR: JDK-8259283: use new HtmlId and HtmlIds classes [v3]

[Jonathan Gibbons]

On Wed, 13 Jan 2021 15:03:17 GMT, Hannes Wallnöfer <hannesw at openjdk.org> wrote:

>> Jonathan Gibbons has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   tidy up merge
>
> src/jdk.javadoc/share/classes/jdk/javadoc/internal/doclets/formats/html/markup/HtmlId.java line 34:
> 
>> 32:  * @see HtmlTree#setId(HtmlId)
>> 33:  */
>> 34: public interface HtmlId {
> 
> Is there a reason for making this an interface instead of a plain class?

At one point, I had in mind using an enum class for the set of fixed ids, and so wanted an interface that I could mix in to the declaration of that class.

Eventually, when we have value types, `HtmlId` could be a good candidate for that feature.

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PR: https://git.openjdk.java.net/jdk/pull/1951