RFR: 8274433: All Cells: misbehavior of startEdit

[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.

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Commit messages:
 - 8274433: All Cells: misbehavior of startEdit

Changes: https://git.openjdk.java.net/jfx/pull/636/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=636&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274433
  Stats: 107 lines in 8 files changed: 107 ins; 0 del; 0 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