RFR: 8252935: Add treeShowing listener only when needed [v6]

[John Hendrikx]

> 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.

John Hendrikx has updated the pull request incrementally with one additional commit since the last revision:

  Add missed null checks to fix test failure

-------------

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/185/files
  - new: https://git.openjdk.java.net/jfx/pull/185/files/02b4272e..7a685f43

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=185&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=185&range=04-05

  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 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