RFR: 8265206: Tree-/TableCell: editing state not updated on cell re-use

[Jeanette Winzenburg]

Issue is missing update of Tree-/TableCell's editiable state when changing its index from editingIndex to -1. 

Seems to be a left-over from fixing cell's editing update - done in [JDK-8150525](https://bugs.openjdk.java.net/browse/JDK-8150525) - on index change for the special case of new index -1.

Fixed by cleaning out editing state in that corner case also, added tests that were failing before and passing after the fix. Note that there are also tests that passed before: the previous fix didn't add any tests, so added them here.

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Commit messages:
 - 8265206: Tree-/TableCell: editing state not updated on cell re-use

Changes: https://git.openjdk.java.net/jfx/pull/473/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=473&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265206
  Stats: 430 lines in 4 files changed: 410 ins; 16 del; 4 mod
  Patch: https://git.openjdk.java.net/jfx/pull/473.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/473/head:pull/473

PR: https://git.openjdk.java.net/jfx/pull/473