RFR: 8290037: Bindings should clean up after themselves when their weak listeners go out of scope [v2]

[John Hendrikx]

> This is an initial (incomplete) implementation of 8290037 for evaluation.
> 
> If the approach is agreed, I will modify the rest of the `*PropertyBase` classes which use weak listeners, and add some tests.
> 
> I didn't use `Cleaner` because cleaning up a listener may block.

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

  Made accidental public field private

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/827/files
  - new: https://git.openjdk.org/jfx/pull/827/files/c2a15137..e2e516a6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=827&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=827&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/827.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/827/head:pull/827

PR: https://git.openjdk.org/jfx/pull/827