RFR: 8339603: Seal the class hierarchy of Node, Camera, LightBase, Shape, Shape3D

[Michael Strauß]

None of these classes can be extended by user code, and any attempt to do so will fail at runtime with an exception. For this reason, we can seal the class hierarchy and remove the run-time checks to turn this into a compile-time error instead.

In some cases, `Node` and `Shape` are extended by JavaFX classes in other modules, preventing those derived classes from being permitted subclasses. A non-exported `AbstractNode` and `AbstractShape` class is provided just for these scenarios. Note that introducing a new superclass is a source- and binary-compatible change (see [JLS ch. 13](https://docs.oracle.com/javase/specs/jls/se22/html/jls-13.html)).

I'm not sure if this change requires a CSR, as it doesn't change the specification in any meaningful way. There can be no valid JavaFX program that is affected by this change.

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Commit messages:
 - Seal Node, Camera, LightBase, Shape, Shape3D

Changes: https://git.openjdk.org/jfx/pull/1556/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1556&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339603
  Stats: 190 lines in 46 files changed: 89 ins; 18 del; 83 mod
  Patch: https://git.openjdk.org/jfx/pull/1556.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1556/head:pull/1556

PR: https://git.openjdk.org/jfx/pull/1556