RFR: 8318841: macOS: Memory leak with MenuItem when Menu.useSystemMenuBar(true) is used

[Johan Vos]

When the Java layer removes a systemmenu, release the native resources related to this systemmenu.
This removes the strong JNI Global ref, which prevents its references from being gc'ed.

The current implementation for the mac-specific system menu creates a menu, but never releases its resources. In the `dealloc` of this menu, the strong jni refs are deleted.
With this PR, we now release the native resources associated with a menuItem when that one is removed from a menu. A consequence is that this menuItem should never be used after being removed from its current menu (e.g. it should not be re-added, or its text/shortcut should not be altered). 
The current implementation will create a new MacMenuDelegate every time a menuItem is inserted into a menu, so there should be no references to the native resources lingering.

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Commit messages:
 - When the Java layer removes a systemmenu, release the native resources related to this systemmenu.

Changes: https://git.openjdk.org/jfx/pull/1277/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1277&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318841
  Stats: 3 lines in 2 files changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1277.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1277/head:pull/1277

PR: https://git.openjdk.org/jfx/pull/1277