RFR: 8335131: Test "javax/swing/JColorChooser/Test6977726.java" failed on Ubuntu Linux 24.04 x64 because the user didn not see the title "Preview" as described.

[Abhishek Kumar]

This test was modified under [JDK-8328403](https://bugs.openjdk.org/browse/JDK-8328403) bug to remove the applet usage. Test modification includes the instruction change as well which says that `Check that there is a panel with "Text Preview Panel" text and with title "Preview" in the JColorChooser`. Applet test didn't say explicitly about the panel title.

When the test is run with `GTKLookAndFeel` option, `Preview` title is missing due to the `showPreviewPanelText` property defined for `GTKLookAndFeel` is set to **FALSE**. `showPreviewPanelText` property is not defined for other L&Fs and that leads to fallback to the default value of **TRUE** for the property while lookup.

Proposed fix is to change the default value of `showPreviewPanelText` property to **TRUE** for GTK L&F.

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Commit messages:
 - JColorChooser Preview Panel Title Fix

Changes: https://git.openjdk.org/jdk/pull/20104/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20104&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335131
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/20104.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20104/head:pull/20104

PR: https://git.openjdk.org/jdk/pull/20104