RFR: 8187309: TreeCell must not change tree's data

[Jeanette Winzenburg]

Issue was TreeView commit editing implementation violated the spec'ed mechanism:

- no default commit handler on TreeView
- TreeCell modifying the data directly

Fix is to move the saving of the edited value from cell into a default commit handler in tree and set that handler in the constructor.

Added tests that failed/passed before/after the fix (along with a sanity test for default commit that passed also before). Also fixed a test bug (incorrect registration of custom commit handler, see [JDK-8280951)](https://bugs.openjdk.java.net/browse/JDK-8280951) in TreeViewTest.test_rt_29650 to keep it passing.

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Commit messages:
 - 8187309: TreeCell must not change tree's data

Changes: https://git.openjdk.java.net/jfx/pull/724/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=724&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8187309
  Stats: 95 lines in 4 files changed: 89 ins; 4 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/724.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/724/head:pull/724

PR: https://git.openjdk.java.net/jfx/pull/724