RFR: 8278425: TreeTableCellStartEditTest uses deprecated TreeTableCell methods

[Marius Hanl]

This PR replaces 2 deprecations which were added in https://bugs.openjdk.java.net/browse/JDK-8188026

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Commit messages:
 - 8278425: TreeTableCellStartEditTest uses deprecated TreeTableCell methods

Changes: https://git.openjdk.java.net/jfx/pull/692/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=692&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278425
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/692.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/692/head:pull/692

PR: https://git.openjdk.java.net/jfx/pull/692