RFR: 8198402: ToggleButton.setToggleGroup causes memory leak when button is removed via ToggleGroup.getToggles()

[Jesper Skov]

Make the two ways of associating a ToggleButton with a ToggleGroup interact correctly.

This fixes https://bugs.openjdk.java.net/browse/JDK-8198402

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Commit messages:
 - 8198402: Fix ToggleButton leak in ToggleGroup

Changes: https://git.openjdk.java.net/jfx/pull/167/files
 Webrev: https://webrevs.openjdk.java.net/jfx/167/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8198402
  Stats: 117 lines in 3 files changed: 102 ins; 10 del; 5 mod
  Patch: https://git.openjdk.java.net/jfx/pull/167.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/167/head:pull/167

PR: https://git.openjdk.java.net/jfx/pull/167