RFR: 8269115: WebView paste event contains old data

[Jay Bhaskar]

Issue: The DataObject uses m_availMimeTypes as Vector of strings, and appending mime types in pasteboard operation like setPlainText, Hence it will append duplicate mime types in m_availMimeTypes , in this case the length clipboardData.types would be wrong, and duplicates data would be copied to clipboard target.
Solution: Use ListHashSet data Structure instead of Vector for m_availMimeTypes.

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Commit messages:
 - 8269115:WebView paste event contains old data

Changes: https://git.openjdk.java.net/jfx/pull/729/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=729&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269115
  Stats: 26 lines in 3 files changed: 12 ins; 3 del; 11 mod
  Patch: https://git.openjdk.java.net/jfx/pull/729.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/729/head:pull/729

PR: https://git.openjdk.java.net/jfx/pull/729