RFR: 8244075: Accelerator of ContextMenu's MenuItem is not removed when ContextMenu is removed from Scene

[Ambarish Rapte]

Issue:
There are several issues related to Accelerator of MenuItem of a ConextMenu set on Control.
1. Accelerator of ContextMenu's MenuItem is not removed when ContextMenu is removed from Scene
2. Accelerator is not updated correctly when the Control is removed from a Scene and Added to a different Scene
3. Accelerator is not removed from Scene when the anchor node is removed from Scene and then it's ContextMenu is set to null

Fix:
The accelerator should be added or removed correctly according to the Scene property of the anchor node. 
The issue#3 in above list is only fixed for Button and fails for other Controls. A test is added for this scenario and I shall report a new issue to address it.
Added tests that fails before and pass after the fix.

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Commit messages:
 - corrrections
 - Correct the listener

Changes: https://git.openjdk.java.net/jfx/pull/515/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=515&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8244075
  Stats: 125 lines in 2 files changed: 112 ins; 12 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/515.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/515/head:pull/515

PR: https://git.openjdk.java.net/jfx/pull/515