RFR: 8377350: [iOS] Add support for UIWindowScene [v3]

[Jose Pereda]

> This PR adds support for iOS native window scene, which was added some time ago starting iOS 13 (in 2019).
> 
> Current iOS development requires iOS SDK 26, and minimum deployment target is set to iOS 15 (meaning that apps will run on all devices with iOS 15 or greater).
> 
> It is safe then to not include the safe-guard `if (@available(iOS 13.0, *)) {...}`, which will make the code more complex.
> 
> Adding the native UIScene and UIWindowScene APIs follows the Apple standards, but as a fallback, the deprecated API is still kept there (`keyWindow` was deprecated starting iOS 13, and `initWithFrame:` was deprecated starting iOS 26). This is for a follow-up issue.
> 
> Note: This patch doesn't really add support for multiple windows, as this would require more complex changes.

Jose Pereda has updated the pull request incrementally with one additional commit since the last revision:

  Update comment, and rename variable

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/2072/files
  - new: https://git.openjdk.org/jfx/pull/2072/files/a392b869..14f1691f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=2072&range=02
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=2072&range=01-02

  Stats: 10 lines in 1 file changed: 1 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jfx/pull/2072.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/2072/head:pull/2072

PR: https://git.openjdk.org/jfx/pull/2072