RFR: 8265210: TreeCell: cell editing state not updated on cell re-use [v2]

[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.

Jeanette Winzenburg has updated the pull request incrementally with one additional commit since the last revision:

  removed commented imports

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/501/files
  - new: https://git.openjdk.java.net/jfx/pull/501/files/84823ef4..9c0827b9

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=501&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=501&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 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