RFR: JDK-8245058: improve presentation of annotations for modules and packages [v2]

[Hannes Wallnöfer]

> This PR adds a dedicated signature to module and package overview pages and moves all code for generating signatures to a new `doclets.formats.html.Signatures` utility class. The `Signatures` class contains two public static methods for module and package signatures as well as two inner classes for type and member signatures. 
> 
> Signatures are rendered as `<div>` elements with top level CSS classes that identify the kind of signature: `module-signature`, `package-signature`, `type-signature` and `member-signature`. Signature components are rendered as `<span>` elements with CSS classes that identify the component, e.g.: `modifiers`, `annotations`, `element-name`, `exceptions`. 
> 
> As a side benefit, this PR reduces the number of methods in `HtmlDocletWriter` used to generate annotation information.

Hannes Wallnöfer has updated the pull request incrementally with one additional commit since the last revision:

  Fix some of the issues brought up in review

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/966/files
  - new: https://git.openjdk.java.net/jdk/pull/966/files/bd2ef09b..68ec5845

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=966&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=966&range=00-01

  Stats: 4 lines in 2 files changed: 0 ins; 1 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/966.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/966/head:pull/966

PR: https://git.openjdk.java.net/jdk/pull/966