RFR: 8256821: TreeViewSkin/Behavior: misbehavior on switching skin

[Jeanette Winzenburg]

issues with behavior:
- memory leak due to an key eventHandler that's not removed
- after dispose, still modifying treeView (anchor) state due to listeners selection that are not removed

issues with skin:
- memory leak due to behavior leaking
- memory leak due to cellFactory in flow not removed
- throws NPE after switching (on modifying root children, refresh) due to listeners not removed

Fixed by cleaning up as needed. Added tests that are failing before and passing after the fix.

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

Commit messages:
 - 8256821: TreeViewSkin/Behavior: misbehavior on switching skin

Changes: https://git.openjdk.java.net/jfx/pull/358/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=358&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256821
  Stats: 158 lines in 6 files changed: 142 ins; 13 del; 3 mod
  Patch: https://git.openjdk.java.net/jfx/pull/358.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/358/head:pull/358

PR: https://git.openjdk.java.net/jfx/pull/358