RFR: JDK-8254721: Improve support for conditionally generated files

[Jonathan Gibbons]

The overall pages is very dynamic and depends on a lot of factors, such as the input files and the command-line
options. Some of the summary files depend on the content of the classes being documented, such as:

* any constant values declared in the files
* any deprecated elements
* any serializable classes
* any classes whose description references system properties

This change is a step towards handling these files in a uniform manner.

There is a new test to test the consistency of the new behavior..

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Commit messages:
 - Merge master
 - fix trailing whitespace
 - JDK-8254721

Changes: https://git.openjdk.java.net/jdk/pull/674/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=674&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254721
  Stats: 274 lines in 25 files changed: 214 ins; 16 del; 44 mod
  Patch: https://git.openjdk.java.net/jdk/pull/674.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/674/head:pull/674

PR: https://git.openjdk.java.net/jdk/pull/674