RFR: 8335131: Test "javax/swing/JColorChooser/Test6977726.java" failed on ubuntu x64 because "Preview" title is missing for GTK L&F [v6]

[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.

Abhishek Kumar has updated the pull request incrementally with one additional commit since the last revision:

  minor change

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20104/files
  - new: https://git.openjdk.org/jdk/pull/20104/files/666c1f4e..457719de

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20104&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20104&range=04-05

  Stats: 2 lines in 1 file 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