RFR: 8247576: Labeled/SkinBase: misbehavior on switching skin

[Jeanette Winzenburg]

Cleanup of LabeledSkinBase to allow for switching skins

- removed null check in listener on graphic's layoutBounds
- added removal of listener in dispose

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Commit messages:
 - 8247576: Labeled/SkinBase: misbehavior on switching skin

Changes: https://git.openjdk.java.net/jfx/pull/355/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=355&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8247576
  Stats: 147 lines in 2 files changed: 146 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/355.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/355/head:pull/355

PR: https://git.openjdk.java.net/jfx/pull/355