RFR: 8318841: macOS: Memory leak with MenuItem when Menu.useSystemMenuBar(true) is used [v6]

[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.

Johan Vos has updated the pull request incrementally with one additional commit since the last revision:

  Use only 10 cycles instead of 50 (preventing this test to take 50 seconds in case it fails).

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1277/files
  - new: https://git.openjdk.org/jfx/pull/1277/files/1432485f..cbe5ea46

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1277&range=05
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1277&range=04-05

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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