RFR: 8188027: List/TableCell: must not fire event in startEdit if already editing

[Jeanette Winzenburg]

issue was a missing check in startEdit if already editing (as Tree- and TreeTableCell do)

fix was to add the check and backout if true, added tests to List/TableCell that fail/pass before/after the fix

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Commit messages:
 - fixed trailing whitespace
 - 8188027: List/TableCell: must not fire event in startEdit if already

Changes: https://git.openjdk.java.net/jfx/pull/571/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=571&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8188027
  Stats: 35 lines in 4 files changed: 35 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/571.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/571/head:pull/571

PR: https://git.openjdk.java.net/jfx/pull/571