RFR: 8305925: Add concatenated list view

[Michael Strauß]

The JavaFX collections framework offers the `FXCollections.concat(ObservableList...)` method, which creates a new modifiable list that contains a concatenated snapshot of the source lists, backed by an `ArrayList`. However, when the source lists are changed, the concatenated list is not updated.

This PR adds `FXCollections.concatenatedObservableList(ObservableList...)`, which creates an _unmodifiable collection view_ onto the source lists that represents the concatenation of all source elements. When any of the source lists are changed, the change is also reflected in the concatenated view.

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Commit messages:
 - Added concatenated observable list

Changes: https://git.openjdk.org/jfx/pull/1087/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1087&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305925
  Stats: 459 lines in 3 files changed: 448 ins; 3 del; 8 mod
  Patch: https://git.openjdk.org/jfx/pull/1087.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1087/head:pull/1087

PR: https://git.openjdk.org/jfx/pull/1087