RFR: 8269081: Tree/ListViewSkin: must remove flow on dispose

[Jeanette Winzenburg]

left-over issue from cleanup of Tree/ListViewSkin: direct children that have been added by the skin must be removed in dispose

fixed by removing the flow (which allowed to revert the previous cleanup of event handlers/cellfactory)

added tests to verify
- constant child count after replacing skin
- no memory leak when switching skin while showing

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Commit messages:
 - 826908: Tree/ListViewSkin: must remove flow on dispose

Changes: https://git.openjdk.java.net/jfx/pull/612/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=612&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269081
  Stats: 90 lines in 3 files changed: 68 ins; 18 del; 4 mod
  Patch: https://git.openjdk.java.net/jfx/pull/612.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/612/head:pull/612

PR: https://git.openjdk.java.net/jfx/pull/612