RFR: JDK-8264274: Block tags in overview.html are ignored

[Hannes Wallnöfer]

This is a simple fix to generate block tags in overview files specified by the `-overview` option. 

I added a protected `addOverviewTags` method in `AbstractOverviewIndexWriter` which is probably overkill but keeps things flexible and separated.

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Commit messages:
 - JDK-8264274: Block tags in overview.html are ignored

Changes: https://git.openjdk.java.net/jdk/pull/5099/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5099&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264274
  Stats: 30 lines in 3 files changed: 20 ins; 1 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5099.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5099/head:pull/5099

PR: https://git.openjdk.java.net/jdk/pull/5099