[jfx11u] RFR: 8216377: JavaFX: memoryleak for initial nodes of Window

[Johan Vos]

8207837: Indeterminate ProgressBar does not animate if content is added after scene is set on window

Add or remove the windowShowingChangedListener when the scene changes

Reviewed-by: arapte, kcr

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Commit messages:
 - 8216377: Fix memoryleak for initial nodes of Window

Changes: https://git.openjdk.java.net/jfx11u/pull/23/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx11u&pr=23&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8216377
  Stats: 120 lines in 2 files changed: 119 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx11u/pull/23.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx11u pull/23/head:pull/23

PR: https://git.openjdk.java.net/jfx11u/pull/23