RFR: 8276141: XPathFactory set/getProperty method

[Joe Wang]

Add setProperty/getProperty methods to the XPath API so that properties can be supported in the future.

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Commit messages:
 - 8276141: XPathFactory set/getProperty method

Changes: https://git.openjdk.java.net/jdk/pull/6215/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6215&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276141
  Stats: 51 lines in 1 file changed: 51 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6215.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6215/head:pull/6215

PR: https://git.openjdk.java.net/jdk/pull/6215