RFR: 8290844: Add Skin.install() method [v6]

[Andy Goryachev]

> - added Skin.install()
> - javadoc changes for Skinnable.setSkin(Skin)
> 
> no code changes for Skinnable.setSkin(Skin) yet.

Andy Goryachev has updated the pull request incrementally with one additional commit since the last revision:

  8290844: review comments

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Changes:
  - all: https://git.openjdk.org/jfx/pull/845/files
  - new: https://git.openjdk.org/jfx/pull/845/files/7d5db78b..e4310ce0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=845&range=05
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=845&range=04-05

  Stats: 4 lines in 2 files changed: 1 ins; 0 del; 3 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