RFR: JDK-8232644: bugs in serialized-form.html

[Hannes Wallnöfer]

This implements most suggestions implemented in the JBS issue. The heading for class entries in the page is shortened to "<element-type-kind> <name>", followed by a simplified signature line like "class X extends Y implements Serializable".

Furthermore, package names in package headings now link to the package page, `@param` info is added to the methods, Externalizable is used for classes that implement it. I also added a check for content in `@serial` tags to avoid spurious empty ` ` lines in the output.

I increased the font size for headings `h4` to `h6` in the stylesheets because their size was smaller than the default text size, which looked strange. Although the change in font size from `h3` to `h4` and beyond is now smaller I think it's still recognizable. Headings `h4` to `h5` are used very little in Javadoc (mostly static doc files and the serialized-form page). I looked at some of the static files and they looked good to me with the new larger heading fonts.

Although I didn't end up using the `Signatures` class to generate the class signatures (they're just too simple and too specific in what they list), I left in some cleanup of `Signatures.TypeSignature`, most significantly moving the processing of modifiers from `ClassWriterImpl` to `Signatures.TypeSignature`.

I only removed the obsolete resources from the English resource file out of habit of not touching the Chinese and Japanese ones. I guess I could remove it there too.

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Commit messages:
 - JDK-8232644: bugs in serialized-form.html

Changes: https://git.openjdk.java.net/jdk/pull/3817/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3817&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8232644
  Stats: 178 lines in 19 files changed: 56 ins; 43 del; 79 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3817.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3817/head:pull/3817

PR: https://git.openjdk.java.net/jdk/pull/3817