RFR: 8271484: Tree-/TableCell: NPE when accessing edit event from startEdit

[Jeanette Winzenburg]

The NPE was an indirect effect: 

- the bug on part of the cell was an incorrect (== missing) edit location in cellEditEvent - that's fixed in this PR
- a related bug is on part of the implementation of CellEditEvent (assuming that TablePosition != null, [JDK-8269871](https://bugs.openjdk.java.net/browse/JDK-8269871) which is not addressed in this PR

Fix is to use the cell's state at startEdit for creating the editing location, added tests starting the edit on the cell that failed/passed before/after the fix, also added sanity tests with starting edit on the control that passed before/after.

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

Commit messages:
 - 8271484: Tree-/TableCell: NPE when accessing edit event from startEdit

Changes: https://git.openjdk.java.net/jfx/pull/592/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=592&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271484
  Stats: 57 lines in 4 files changed: 53 ins; 2 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/592.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/592/head:pull/592

PR: https://git.openjdk.java.net/jfx/pull/592