RFR: 8269907 memory leak - Dirty Nodes / Parent removed [v7]

[Florian Kirmaier]

> After thinking about this issue for some time, I've now got a solution.
> I know put the scene in the state it is, before is was shown, when the dirtyNodes are unset, the whole scene is basically considered dirty. 
> This has the drawback of rerendering, whenever a window is "reshown", but it restores sanity about memory behaviour, which should be considered more important.

Florian Kirmaier has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains eight commits:

 - JDK-8269907
   Added missing changes after merge
 - Merge remote-tracking branch 'origjfx/master' into JDK-8269907-dirty-and-removed
   
   # Conflicts:
   #	modules/javafx.graphics/src/main/java/com/sun/javafx/tk/Toolkit.java
   #	modules/javafx.graphics/src/main/java/javafx/scene/Scene.java
 - Merge remote-tracking branch 'origin/master'
 - JDK-8269907
   Removed the sync methods for the scene, because they don't work when peer is null, and they are not necessary.
 - JDK-8269907
   Fixed rare bug, causing bounds to be out of sync.
 - JDK-8269907
   We now require the rendering lock when cleaning up dirty nodes. To do so, we moved some code required for snapshot into a reusable method.
 - JDK-8269907
   The bug is now fixed in a new way. Toolkit now supports registering CleanupListeners, which can clean up the dirty nodes, avoiding memoryleaks.
 - JDK-8269907
   Fixing dirty nodes and parent removed, when a window is no longer showing.  This typically happens with context menus.

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Changes: https://git.openjdk.org/jfx/pull/584/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=584&range=06
  Stats: 130 lines in 3 files changed: 124 ins; 2 del; 4 mod
  Patch: https://git.openjdk.org/jfx/pull/584.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/584/head:pull/584

PR: https://git.openjdk.org/jfx/pull/584