RFR: JDK-8307807 Replace use of System.getProperty("os.name") with PlatformUtil calls [v6]

[Carl Döbbelin]

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

Carl Döbbelin has updated the pull request incrementally with one additional commit since the last revision:

  made launcherimpl one line

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1132/files
  - new: https://git.openjdk.org/jfx/pull/1132/files/97dfdb99..dc6630cf

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1132&range=05
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1132&range=04-05

  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 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