RFR: 8371070: RichParagraph enhancements [v9]

[Andy Goryachev]

> This PR addresses the user feedback:
> 
> In RichParagraph you have a note to make getSegments() public.  It would be helpful if it was.
> 
> 
> The problem being solved here is to allow the developers to easily add functionality to the existing `RichTextModel` (and its storage implementation) without creating a new model from scratch.
> 
> This is done in two parts:
> 
> 1. Allowing the custom model to alter the paragraph by giving access to its Builder via a new protected method
> 
> 
> protected RichParagraph.Builder buildParagraph(int index)
> 
> 
> 2. Adding two methods that give access to the segments in the `RichParagraph` and `RichParagraph.Builder` classes:
> 
> - getSegmentCount()
> - getSegment(int index)
> 
> NOTE: Even though returning an (immutable) List might be considered "more useful", doing so would necessitate creating a copy - something I do want to avoid.

Andy Goryachev has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 14 commits:

 - prepare paragraph
 - Merge branch 'master' into 8371070.enhance
 - javadoc
 - cleanup
 - Merge branch 'master' into 8366198.rbuilder
 - rich paragraph builder api
 - Merge branch 'master' into 8371070.enhance
 - npe
 - merge
 - Merge branch 'master' into 8371070.enhance
 - ... and 4 more: https://git.openjdk.org/jfx/compare/b39e1faa...dd3ec196

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Changes: https://git.openjdk.org/jfx/pull/1966/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1966&range=08
  Stats: 275 lines in 8 files changed: 198 ins; 43 del; 34 mod
  Patch: https://git.openjdk.org/jfx/pull/1966.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1966/head:pull/1966

PR: https://git.openjdk.org/jfx/pull/1966