RFR: 8290844: Add Skin.install() method [v9]

[Johan Vos]

On Mon, 3 Oct 2022 18:13:25 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

> @johanvos Do you have any thoughts on this enhancement? I'm most interested whether you extend any of the built-in JavaFX skin classes in a way that it might cause problems for you if they move some of their setup code from their constructor to their `install` method?

We did a quick internal sanity check, and this won't cause any problems for us. I've read through the comments in the discussions here, and I understand the point "If it ain't broken (badly), don't fix (change) it", but my main thought is that this seems to be our opportunity to fix a design issue. 
Unless I'm missing something, the refactory efforts for cases that extend build-in skins should be fairly minimal. 
I added a few comments, mainly related to the javadocs as those should be crystal clear for developers, leaving no room for creative implementations.

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