RFR: 8252935: Add treeShowing listener only when needed

Wed Sep 9 21:35:56 UTC 2020

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.

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Commit messages:
 - WIP: Moved treeShowingProperty into its own class

Changes: https://git.openjdk.java.net/jfx/pull/185/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=185&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252935
  Stats: 275 lines in 6 files changed: 161 ins; 109 del; 5 mod
  Patch: https://git.openjdk.java.net/jfx/pull/185.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/185/head:pull/185

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