RFR: 8251946: ObservableList.setAll does not conform to specification

[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.)

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Commit messages:
 - 8251946: Simplify ModifiableObservableListBase#setAll and only special-case two empty lists
 - 8251946: Correctly compute if the list was modified in ModifiableObservableListBase#setAll

Changes: https://git.openjdk.java.net/jfx/pull/284/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=284&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8251946
  Stats: 23 lines in 3 files changed: 20 ins; 2 del; 1 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