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

[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.

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Commit messages:
 - 8187229: Tree/TableCell: cancel event must return correct editing

Changes: https://git.openjdk.java.net/jfx/pull/561/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=561&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8187229
  Stats: 401 lines in 7 files changed: 374 ins; 15 del; 12 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