[Rev 02] RFR: 8236912: NullPointerException when clicking in WebView with Button 4 or Button 5

[Robert Lichtenberger]

> As documented in JDK-8236912, WebView did not check whether the idMap really contained a mapping for the given button, making it prone to errors, when things are extended (as has happened here).
> 
> The fix consists of two test cases that show the problem in unfixed WebViews and a fix which works analogously to the check whether the given event type is mapped.

The pull request has been updated with a new target base due to a merge or a rebase.

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Commits:
 - 8c3f6017: 8232524: SynchronizedObservableMap cannot be be protected for copying/iterating
 - 829920cf: JDK-8232524 fixed.
 - 401b4484: Fix null pointer exception when clicking into WebViews with forward/back mouse button.
 - 3bf7e920: 8236733: Change JavaFX release version to 15
 - 0967bbd3: 8232524: Test cleanup: terminate background thread upon failure.
 - e50b0533: 8232524: SynchronizedObservableMap cannot be be protected for copying/iterating
 - 9d1979e7: JDK-8232524 fixed.

Changes: https://git.openjdk.java.net/jfx/pull/85/files
 Webrev: https://webrevs.openjdk.java.net/jfx/85/webrev.02
  Stats: 205 lines in 6 files changed: 198 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jfx/pull/85.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/85/head:pull/85

PR: https://git.openjdk.java.net/jfx/pull/85