<AWT Dev> RFR: 8256109: Create implementation for NSAccessibilityButton protocol

[Alexander Zuev]

Creating a native peer for pushbutton accessibility role on Max OS X.
The process of choosing of what peer to initialize is clunky so
might be addressed in the future.

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Commit messages:
 - JDK-8256109: Create implementation for NSAccessibilityButton protocol
 - Merge pull request #5 from openjdk/master
 - Merge pull request #4 from openjdk/master
 - Merge pull request #3 from openjdk/master
 - Merge pull request #2 from openjdk/master
 - Merge pull request #1 from openjdk/master

Changes: https://git.openjdk.java.net/jdk/pull/1549/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1549&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256109
  Stats: 33 lines in 1 file changed: 33 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1549.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1549/head:pull/1549

PR: https://git.openjdk.java.net/jdk/pull/1549