RFR: 8252935: Add treeShowing listener only when needed

[Kevin Rushforth]

On Fri, 17 Apr 2020 08:06:23 GMT, John Hendrikx <jhendrikx at openjdk.org> wrote:

> This is a PoC for performance testing.
> 
> It contains commented out code in PopupWindow and ProgressIndicatorSkin and two tests are failing because of that.
> 
> This code avoids registering two listeners (on Scene and on Window) for each and every Node to support the
> aforementioned classes.  The complete change should update these two classes to add their own listeners instead.

These listeners exist for a good reason. Removing them will almost certainly cause regressions in behavior. See
[JDK-8090322](https://bugs.openjdk.java.net/browse/JDK-8090322) as well as the follow-on fixes (since that was a rather
involved fix in the first place).

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