RFR: 8256397: MultipleSelectionModel throws IndexOutOfBoundException [v6]

[Florian Kirmaier]

> Fixing IndexOutOfBoundsException in the MultipleSelectionModelBase and added a unit-test for it.
> ticket: https://bugs.openjdk.java.net/browse/JDK-8256397
> run test: `./gradlew --continue -PFULL_TEST=true controls:test --tests "*MultipleSelectionModelImplTest*"`

Florian Kirmaier has updated the pull request incrementally with one additional commit since the last revision:

  JDK_8256397
  Some changes based on codereview

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/353/files
  - new: https://git.openjdk.org/jfx/pull/353/files/833afbb1..991b114f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=353&range=05
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=353&range=04-05

  Stats: 19 lines in 2 files changed: 2 ins; 0 del; 17 mod
  Patch: https://git.openjdk.org/jfx/pull/353.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/353/head:pull/353

PR: https://git.openjdk.org/jfx/pull/353