RFR: 8265210: TreeCell: cell editing state not updated on cell re-use

[Jeanette Winzenburg]

fix is analogous to similar issues for Tree/TableCell (combined [JDK-8150525](https://bugs.openjdk.java.net/browse/JDK-8150525) and [JDK-8265206](https://bugs.openjdk.java.net/browse/JDK-8265206))

added tests that failed before and passed after the fix

note: this PR also adds two asserts (which I forgot in the previous [PR 473](https://github.com/openjdk/jfx/pull/473);) in Tree/TableEditingTest which are unrelated to this fix.

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Commit messages:
 - 8265210: TreeCell: cell editing state not updated on cell re-use

Changes: https://git.openjdk.java.net/jfx/pull/501/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=501&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265210
  Stats: 385 lines in 5 files changed: 354 ins; 17 del; 14 mod
  Patch: https://git.openjdk.java.net/jfx/pull/501.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/501/head:pull/501

PR: https://git.openjdk.java.net/jfx/pull/501