RFR: 8252935: Add treeShowing listener only when needed [v3]

[Ambarish Rapte]

On Mon, 1 Feb 2021 04:41:20 GMT, John Hendrikx <jhendrikx at openjdk.org> wrote:

>> The updated fix using `registerChangeListener` and the test look good. I left a few mostly-minor comments.
>
> I think this change is complete now, including an integration test and tests on all new code.  Please let me know if anything else needs to be done.

This PR also seems to fix another issue [JDK-8259558](https://bugs.openjdk.java.net/browse/JDK-8259558).  Test program is attached to the bug.

Time readings,
with this fix,
time to remove 100000 nodes : ~20 ms
time to add those removed 100000 nodes to a different parent:  ~80 ms

without this fix,
time to remove 100000 nodes : ~1720 ms
time to add those removed 100000 nodes to a different parent:  ~100 ms

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PR: https://git.openjdk.java.net/jfx/pull/185