RFR: JDK-8187314: All Cells: must show backing data always [v2]

[Marius Hanl]

> Before, the `updateItem` method was called with the new value that was committed via `commitEdit()`.
> This is problematic as developers may setup a commit handler via `setOnEditCommit`, which may reject the edit (or change the value otherwise).
> We therefore do call the `updateItem(-1)` which will also call `updateItem` but with the real underlying value.
> 
> Changed and added tests for all 4 cells.

Marius Hanl has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge remote-tracking branch 'origin/master' into 8187314-cell-backing-data
 - JDK-8187314: All Cells: must show backing data always

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1197/files
  - new: https://git.openjdk.org/jfx/pull/1197/files/bcc74881..e36bebc8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1197&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1197&range=00-01

  Stats: 13370 lines in 329 files changed: 10594 ins; 1569 del; 1207 mod
  Patch: https://git.openjdk.org/jfx/pull/1197.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1197/head:pull/1197

PR: https://git.openjdk.org/jfx/pull/1197