[jfx17] RFR: 8270246: Deprecate for removal implementation methods in Scene

[Kevin Rushforth]

The following methods in Scene were former `impl_*` method that were intended to be encapsulated as part of [JDK-8157295](https://bugs.openjdk.java.net/browse/JDK-8157295) in JDK 9, but were mistakenly left as public:


    public void disposePeer()
    public void enableInputMethodEvents(boolean enable)
    public void processKeyEvent(KeyEvent e)


In the fix for [JDK-8157295](https://bugs.openjdk.java.net/browse/JDK-8157295), the `impl_` prefix was removed, and the necessary accessors were added, but the `public` modifier was not removed. See the diffs for these three methods in Scene.java: [`disposePeer`](https://github.com/openjdk/jfx/commit/d864a620dfc7631585872547b42dcf93e9d570e6#diff-72cb5f9e4cc8a3804417166fdef390361e82d5890603f4e5158c9fc70c22e9daL833), [`enableInputMethodEvents`](https://github.com/openjdk/jfx/commit/d864a620dfc7631585872547b42dcf93e9d570e6#diff-72cb5f9e4cc8a3804417166fdef390361e82d5890603f4e5158c9fc70c22e9daL2221), [`processKeyEvent`](https://github.com/openjdk/jfx/commit/d864a620dfc7631585872547b42dcf93e9d570e6#diff-72cb5f9e4cc8a3804417166fdef390361e82d5890603f4e5158c9fc70c22e9daL2129)

We will deprecate these methods for removal with the intention to remove them from the public API in a future version of JavaFX. This will need a CSR.

NOTE: this is targeted to `jfx17`.

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Commit messages:
 - 8270246: Deprecate for removal implementation methods in Scene

Changes: https://git.openjdk.java.net/jfx/pull/573/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=573&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270246
  Stats: 17 lines in 1 file changed: 17 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/573.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/573/head:pull/573

PR: https://git.openjdk.java.net/jfx/pull/573