RFR: JDK-8307807 Replace use of System.getProperty("os.name") with PlatformUtil calls

[Carl Döbbelin]

Replaced querying of `System.getProperty("os.name")` to check for OS with the use of the methods provided by `PlatformUtils`

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Commit messages:
 - Merge branch 'master' into JDK-8307807-replace-system-getproperty-osname-platform-utils
 - replaced sensible spots of system getproperty os name with platform util

Changes: https://git.openjdk.org/jfx/pull/1132/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1132&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307807
  Stats: 37 lines in 5 files changed: 9 ins; 10 del; 18 mod
  Patch: https://git.openjdk.org/jfx/pull/1132.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1132/head:pull/1132

PR: https://git.openjdk.org/jfx/pull/1132