RFR: 8371859: Dialog unnecessarily invokes DialogPane.requestLayout()

[Michael Strauß]

The implementation of `Dialog.dialogPane` adds a `ListChangeListener` to `DialogPane.getButtonTypes()`, and calls `DialogPane.requestLayout()` from the listener.

This is not needed, because the `DialogPane.createButtonBar()` method already specifies that implementers are required to observe changes of `DialogPane.getButtonTypes()`, and relayout the button bar as needed.

I've added a test to verify that changing the button types indeed requests a layout on the `DialogPane`.

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Commit messages:
 - fix

Changes: https://git.openjdk.org/jfx/pull/1973/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1973&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371859
  Stats: 18 lines in 2 files changed: 10 ins; 6 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/1973.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1973/head:pull/1973

PR: https://git.openjdk.org/jfx/pull/1973