RFR: 8252238: TableView: Editable (pseudo-editable) cells should respect the row editability

[Marius Hanl]

This PR enables Tree- and TableCells to also check the row editability when an edit should happen. With this a Tree- or TableCell is not editable, when the row where the cell is in is not.

While this PR fixes the problem described in the ticket, it does not fix the example.
This is due the example uses the **CheckBoxTableCell**, which is a ready-to-use subclass of **TableCell**. 

While looking into this, I found out that multiple sub implementations still have this issue, but the fix is not always the same, e.g. CheckBoxTableCell should disable the CheckBox (in **updateItem**), while the ChoiceBoxTableCell should check the row editability in the **startEdit** method (like this PR does).

I created a follow-up issues for fixing all the sub Tree- and TableCell implementation which do not count the row editability in:
[JDK-8268295](https://bugs.openjdk.java.net/browse/JDK-8268295)

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Commit messages:
 - Table, Tree,-Cells respect the row editability now

Changes: https://git.openjdk.java.net/jfx/pull/529/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=529&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252238
  Stats: 54 lines in 4 files changed: 44 ins; 0 del; 10 mod
  Patch: https://git.openjdk.java.net/jfx/pull/529.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/529/head:pull/529

PR: https://git.openjdk.java.net/jfx/pull/529