RFR: 8241840: Memoryleak: Closed focused Stages are not collected with Monocle. [v6]

[Florian Kirmaier]

> Closed focused Stages are not collected with Monocle
> 
> This commit adds a unit-test and a fix for collecting focused closed stages.
> 
> ticket: https://bugs.openjdk.java.net/browse/JDK-8241840

Florian Kirmaier has updated the pull request incrementally with three additional commits since the last revision:

 - JDK-8241840
   fixed tests
 - JDK-8241840
   Removed unnecessary change.
 - JDK-8241840
   Minor cleanups

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/153/files
  - new: https://git.openjdk.java.net/jfx/pull/153/files/3f618c11..a7c9d83c

Webrevs:
 - full: https://webrevs.openjdk.java.net/jfx/153/webrev.05
 - incr: https://webrevs.openjdk.java.net/jfx/153/webrev.04-05

  Stats: 9 lines in 4 files changed: 3 ins; 4 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/153.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/153/head:pull/153

PR: https://git.openjdk.java.net/jfx/pull/153