RFR: 8342782: AWTEventMulticaster throws StackOverflowError using AquaButtonUI [v6]

[Jeremy]

> The AWTEventMulticaster is a tree node with 2 children. This PR proposes rebalancing that tree after 500 additions to avoid potential StackOverflowErrors when trying to interact with a large AWTEventMulticaster.
> 
> In the original headful test:
> We added 8,000 checkboxes, and when their parent panel was hidden the stack needed to grow to 24,000 lines. It took 8,000 lines to recursively call `java.awt.AWTEventMulticaster.componentHidden`, and then 16,000 additional lines to call two recursive methods to remove a listener:
> 
> java.desktop/java.awt.AWTEventMulticaster.removeInternal()
> java.desktop/java.awt.AWTEventMulticaster.remove()
> 
> 
> With this current PR the max stack size reaches 1,267 instead. (If we rebalanced at EVERY addition, then that same scenario would reach a max stack size of 71.)
> 
> JDK-8342782 included a headful test case, but I think the main problem it demonstrated can be represented by the headless test case attached to this PR.
> 
> Depending on how this PR is received I may submit a separate ticket & PR to modify AquaButtonUI so it doesn't always attach an AncestorListener. (That is: if my GUI includes 8,000 checkboxes then I don't need 8,000 AncestorListeners.) But JDK-8342782's test case is currently written in a way that should reproduce across all L&F's, so that can be discussed separately.

Jeremy has updated the pull request incrementally with five additional commits since the last revision:

 - 8342782: move comments re implementation details inside method
   
   Also avoid restating constant, instead only refer to it as "threshold"
   
   This is in response to:
   https://github.com/openjdk/jdk/pull/21962#discussion_r1855148909
 - 8342782: adding an empty line representing EOF
   
   This is in response to:
   https://github.com/openjdk/jdk/pull/21962#discussion_r1871348260
 - 8342782: changing "/2" to " / 2"
   
   This is in response to:
   https://github.com/openjdk/jdk/pull/21962#discussion_r1871376542
 - 8342782: change threshold from 500 to 100
   
   This is in response to:
   https://github.com/openjdk/jdk/pull/21962#discussion_r1855398578
 - 8342782: convert value (500) to named constant
   
   This is in response to:
   https://github.com/openjdk/jdk/pull/21962#discussion_r1855397860

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/21962/files
  - new: https://git.openjdk.org/jdk/pull/21962/files/ed6a585a..f2212092

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21962&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21962&range=04-05

  Stats: 14 lines in 2 files changed: 6 ins; 5 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/21962.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21962/head:pull/21962

PR: https://git.openjdk.org/jdk/pull/21962