RFR: 8372974: Add missing @Override annotations in "com.sun.java.swing.plaf.gtk" package

[Sergey Bylokhov]

This patch adds missing `@Override` annotations to methods in the `com.sun.java.swing.plaf.gtk` package that override methods from a superclass or interface.
Only source annotations are added; there are no behavioral changes.

The previous patch for `com.sun.beans` can be found here: https://github.com/openjdk/jdk/pull/27887.

BTW, does anybody know why the gtk l&f is located in the shared code? Possibly we could move it to the unix folder, similar to how the windows l&f was moved to the windows folder in https://bugs.openjdk.org/browse/JDK-8189656 ?

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Commit messages:
 - gtkover

Changes: https://git.openjdk.org/jdk/pull/28624/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28624&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372974
  Stats: 198 lines in 9 files changed: 190 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/28624.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28624/head:pull/28624

PR: https://git.openjdk.org/jdk/pull/28624