RFR: 8244112: Skin implementations: must not violate contract of dispose

[Jeanette Winzenburg]

some skins have not been guarding themselves against multiple calls to dispose (see issue for details)

Fixed by backing out off dispose if skinnable is null. Added test (parameterized in control class) for all controls in
the controls package. Those that failed for the misbehaving skins before are passing after the fix.

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Commit messages:
 - 8244112: Skin implementations: must not violate contract of dispose

Changes: https://git.openjdk.java.net/jfx/pull/209/files
 Webrev: https://webrevs.openjdk.java.net/jfx/209/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8244112
  Stats: 241 lines in 7 files changed: 241 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/209.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/209/head:pull/209

PR: https://git.openjdk.java.net/jfx/pull/209