RFR: 8274433: All Cells: misbehavior of startEdit [v2]

[Jeanette Winzenburg]

> The misbehavior happens if (super) startEdit didn't succeed (== !cell.isEditing):
> 
> - must not fire editStart event
> - must not update control's editing location
> 
> fix is to back out of startEdit if super.startEdit doesn't switch the cell into editing mode
> 
> Added tests that failed/passed before/after the fix. Note that for Tree-, Table-, TreeTableCell one of the added tests would be a false green due to those cells not updating the editing location on its control [JDK-8187474](https://bugs.openjdk.java.net/browse/JDK-8187474), so it's ignore until that's fixed.

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

  test cleanup, following review comments

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/636/files
  - new: https://git.openjdk.java.net/jfx/pull/636/files/43015266..e51cb865

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=636&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=636&range=00-01

  Stats: 9 lines in 4 files changed: 0 ins; 2 del; 7 mod
  Patch: https://git.openjdk.java.net/jfx/pull/636.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/636/head:pull/636

PR: https://git.openjdk.java.net/jfx/pull/636