RFR: 8304912: Use OperatingSystem enum in java.desktop module [v2]

[Roger Riggs]

> Update classes in the java.desktop module to use the jdk.internal.util.OperatingSystem enum instead of the `os.name` system property to select OS specific behaviors.

Roger Riggs has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - Merge branch 'master' into 8304912-use-os-desktop
 - Use OperatingSystem.isAix
 - Remove unused os.arch system property
 - Apply OperatingSystem enum instead of os.name to java.desktop module

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13213/files
  - new: https://git.openjdk.org/jdk/pull/13213/files/b8cd2240..7f93ecbf

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13213&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13213&range=00-01

  Stats: 21757 lines in 527 files changed: 9811 ins; 9926 del; 2020 mod
  Patch: https://git.openjdk.org/jdk/pull/13213.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13213/head:pull/13213

PR: https://git.openjdk.org/jdk/pull/13213