RFR: 8283551: ControlAcceleratorSupport menu items listener causes memory leak

[Dean Wookey]

Each time a menu would change scenes, a new set of ListChangeListeners would be added to the items in the menu. The bigger problem however is that these list change listeners have a strong reference to the scene which is potentially a much bigger leak.

The first commit was more straightforward, but there are 2 things that might be of concern:

1. The method removeAcceleratorsFromScene takes in a scene parameter, but it'll remove all the ListChangeListeners attached to it, regardless of which scene was passed in. Something similar happens with changeListenerMap already, so I think it's fine.
2. I made the remove method public so that external calls from skins to remove the accelerators would remove the ListChangeListener and also because all the remove methods are public. 

After I wrote more tests I realised using the ObservableLists as keys in the WeakHashMaps was a bad idea. If Java had a WeakIdentityHashMap the fix would be simple. The fix is in the second commit.

There are still more issues that stem from the fact that for each anchor there could be multiple menus and the current code doesn't account for that. For example, swapping context menus on a tab doesn't register change listeners on the new context menu because the TabPane itself had a scene change listener already. These other issues could relate to JDK-8268374 https://bugs.openjdk.org/browse/JDK-8268374 and JDK-8283449 https://bugs.openjdk.org/browse/JDK-8283449. One of the issues is related to the fact that there's no logic to remove listeners when Tab/TableColumn's are removed from their associated control (TabPane, TableView, TreeTableView).

I'm looking at these issues, but I think they're dependent on this fix. Either I can add to this PR or I can wait to see what comes out of this and fix them subsequently.

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Commit messages:
 - No longer use ObservableList as keys
 - JDK-8283551 ControlAcceleratorSupport menu items listener causes memory leak

Changes: https://git.openjdk.org/jfx/pull/1037/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1037&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8283551
  Stats: 343 lines in 4 files changed: 323 ins; 1 del; 19 mod
  Patch: https://git.openjdk.org/jfx/pull/1037.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/1037/head:pull/1037

PR: https://git.openjdk.org/jfx/pull/1037