RFR: 8290844: Add Skin.install() method

[Andy Goryachev]

- added Skin.install()
- javadoc changes for Skinnable.setSkin(Skin)

no code changes for Skinnable.setSkin(Skin) yet.

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Commit messages:
 - Merge remote-tracking branch 'origin/master' into 8290844.skin.install
 - 8289397: added space
 - 8290844: skin.install
 - 8290844: whitespace
 - 8290844: validate input
 - 8290844: illegal argument exception
 - 8290844: illegal argument exception
 - 8290844: whitespace
 - 8290844: <p>
 - 8290844: add Skin.install()

Changes: https://git.openjdk.org/jfx/pull/845/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=845&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290844
  Stats: 53 lines in 4 files changed: 48 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jfx/pull/845.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/845/head:pull/845

PR: https://git.openjdk.org/jfx/pull/845