RFR: 8301763: Adding children to wrong index leaves inconsistent state in Parent#childrenSet

[Lukasz Kostyra]

This issue happened because `childSet` member of Parent was modified during `onProposedChange()` call - that call did not recognize negative indexes as invalid, which caused an exception when actually adding the Node to a List.

This seemed like the simplest solution which doesn't rework a lot of code underneath. Exceptions coming from a backing list/collection technically are handled by `VetoableListDecorator`'s try-catch clauses, however `VetoableListDecorator` does not provide an interface to react when such an exception happens - without it we cannot revert `childSet` back to its original state.

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Commit messages:
 - Add negative index test for Parent
 - Parent: Throw exception when added child index is negative

Changes: https://git.openjdk.org/jfx/pull/1136/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1136&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301763
  Stats: 22 lines in 2 files changed: 22 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1136.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1136/head:pull/1136

PR: https://git.openjdk.org/jfx/pull/1136