RFR: 8251946: ObservableList.setAll does not conform to specification [v3]

[Leon Linhart]

> Hi, this PR fixes [JDK-8251946](https://bugs.openjdk.java.net/browse/JDK-8251946) computing whether the list was
> actually modified instead of just returning `true`. The list was modified if 1. it was not empty (modified by calling
> `#clear()`), or if 2. it was modified as result of the `#addAll()` call.
> 
> If you want any test coverage for this please let me know.
> 
> I reported the issue a couple of days ago via web formula and waited for the confirmation to open this PR but now I see
> that @kevinrushforth (sorry for pinging) is already assigned to the JBS issue. I'm not too familiar with how you handle
> JBS issues or more specifically whether assigning is used to indicate that the issue is in the assignee's "domain", or
> that the assignee is already working on it. In the latter case, please feel free to close this PR. (My OCA submission
> is still pending anyway.)

Leon Linhart has updated the pull request incrementally with one additional commit since the last revision:

  Reverted incorrect change and improved test coverage

-------------

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/284/files
  - new: https://git.openjdk.java.net/jfx/pull/284/files/e8857913..5f260d7e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=284&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=284&range=01-02

  Stats: 5 lines in 2 files changed: 3 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/284.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/284/head:pull/284

PR: https://git.openjdk.java.net/jfx/pull/284