RFR: JDK-8313799: Remove lockItemOnEdit flag from (Tree)TableCell

[Marius Hanl]

The `lockItemOnEdit` only exists inside `TreeTableCell` and `TableCell` for the sake of testing.
It is only changed by some JUnit tests to remove the need of setting up the whole table framework.
This PR shifts this flag from those two classes to the shim classes, which are not inside the final JavaFX product, only used for testing.

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Commit messages:
 - JDK-8313799: Remove lockItemOnEdit flag from (Tree)TableCell

Changes: https://git.openjdk.org/jfx/pull/1198/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1198&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313799
  Stats: 90 lines in 8 files changed: 50 ins; 21 del; 19 mod
  Patch: https://git.openjdk.org/jfx/pull/1198.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1198/head:pull/1198

PR: https://git.openjdk.org/jfx/pull/1198