[jdk11u-dev] RFR: 8256110: Create implementation for NSAccessibilityStepper protocol

[Autumn Capasso]

Backport-of: a54adefb246d945fee88e19ca38b601ffc628305 for [JDK-8256110](https://bugs.openjdk.org/browse/JDK-8259779) Mostly clean backport. 2/28 of the series of backports.

-------------

Commit messages:
 - 8256110: Create implementation for NSAccessibilityStepper protocol

Changes: https://git.openjdk.org/jdk11u-dev/pull/1544/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=1544&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8256110
  Stats: 404 lines in 6 files changed: 387 ins; 15 del; 2 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/1544.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev pull/1544/head:pull/1544

PR: https://git.openjdk.org/jdk11u-dev/pull/1544