RFR: 8253634: TreeCell/Skin: misbehavior on switching skin

[Jeanette Winzenburg]

TreeCellSkin installs listeners to the TreeView/fixedCellSize that introduce a memory leak, NPE on replacing the
treeView and incorrect update of internal state.

Fixed by removing the listeners (and the internal state had been copied from treeView on change) and access of listView
state when needed.

Added tests that failed before and pass after the fix, plus a sanity test to guarantee same (correct) behavior
before/after.

Issue and fix is basically the same as for ListCellSkin [JDK-8246745](https://bugs.openjdk.java.net/browse/JDK-8246745)

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Commit messages:
 - 8253634: TreeCell/Skin: misbehavior on switching skin

Changes: https://git.openjdk.java.net/jfx/pull/309/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=309&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253634
  Stats: 101 lines in 4 files changed: 60 ins; 36 del; 5 mod
  Patch: https://git.openjdk.java.net/jfx/pull/309.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/309/head:pull/309

PR: https://git.openjdk.java.net/jfx/pull/309