RFR: 8187229: Tree/TableCell: cancel event must return correct editing location [v2]

[Jeanette Winzenburg]

> the bug is an incorrect edit location (for Tree/Table: Tree/TablePosition) in edit cancel events - expected is the location at the time the cell edit was started, actual was the location of at the time the edit was cancelled. See the report for details.
> 
> The fix is analogue to those for ListCell/TreeCell, that is storing the edit location in startEdit and use that in cancelEdit.
> 
> Added tests that failed before and passed after and tests that (accidentally :) passed before and still pass after.
> 
> Related issues:
> 
> - also fixes [JDK-8269136](https://bugs.openjdk.java.net/browse/JDK-8269136) (Tree/TablePosition: must not throw NPE on instantiating with null table), in hind-sight it seemed too small to warrant its own PR. 
> - does not fix the implementation of CellEditEvent (todo: file follow-up issue) which must not throw with null Tree/TablePosition.

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

  test cleanup as per review comments

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/561/files
  - new: https://git.openjdk.java.net/jfx/pull/561/files/2b52e86c..9b9a52e0

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=561&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=561&range=00-01

  Stats: 31 lines in 2 files changed: 0 ins; 27 del; 4 mod
  Patch: https://git.openjdk.java.net/jfx/pull/561.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/561/head:pull/561

PR: https://git.openjdk.java.net/jfx/pull/561