RFR: 8165214: ListView.EditEvent.getIndex() does not return the correct index

[Jeanette Winzenburg]

Issue was that the cancel event carried the listView's editingIndex at the time of firing the event - that's wrong nearly always (because the list's editing state/index might have changed between start and cancel, f.i. due to calling list.edit(someDifferentIndex)). 

Fixed by keeping the index at startEdit and using that in cancelEdit (similar approach as in TreeCell fix [TreeCell fix JDK-8265210](https://bugs.openjdk.java.net/browse/JDK-8265210). 

Added tests that are failing (and one that was accidentally passing) before, all passing after.

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Commit messages:
 - 8165214: ListView.EditEvent.getIndex() does not return the correct index

Changes: https://git.openjdk.java.net/jfx/pull/539/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=539&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8165214
  Stats: 105 lines in 2 files changed: 98 ins; 4 del; 3 mod
  Patch: https://git.openjdk.java.net/jfx/pull/539.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/539/head:pull/539

PR: https://git.openjdk.java.net/jfx/pull/539