RFR: 8175358: Memory leak when moving MenuButton into another Scene

[Ajit Ghaisas]

Issue : https://bugs.openjdk.java.net/browse/JDK-8175358

Root cause : When a MenuItem is removed from a Scene, if any accelerator has been set on MenuItem, it does not get
removed from Scene's list of accelerators.

Fix : If Scene changes for a Menu, remove the registered accelerators from Scene.

Testing : Added a unit test that fails before the fix and passes with it.

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Commit messages:
 - menuItem_accelerator_fix

Changes: https://git.openjdk.java.net/jfx/pull/199/files
 Webrev: https://webrevs.openjdk.java.net/jfx/199/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8175358
  Stats: 46 lines in 2 files changed: 44 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/199.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/199/head:pull/199

PR: https://git.openjdk.java.net/jfx/pull/199