RFR: 8349758: Memory leak in TreeTableView

[Andy Goryachev]

## Root Cause

An event handler was installed on the root property without removing it when the root changes.

## Solution

Replaced the weak parts with a change listener to the root property to ensure correct handling of the root value changes.

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Commit messages:
 - cleanup
 - remove weak listener

Changes: https://git.openjdk.org/jfx/pull/1706/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1706&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349758
  Stats: 105 lines in 1 file changed: 41 ins; 57 del; 7 mod
  Patch: https://git.openjdk.org/jfx/pull/1706.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1706/head:pull/1706

PR: https://git.openjdk.org/jfx/pull/1706