RFR: 8218474: JComboBox display issue with GTKLookAndFeel

[Abhishek Kumar]

In case of GTK LAF, the current selected element was not visible due to arrowbutton is rendered in entire display area of Combobox.

The proposed fix is to set the squareButton property of combobox to false so that the arrowbutton didn't take entire combobox display area for rendering. The getMinimumSize method is overridden in SynthComboBoxUI to consider arrowbutton insets for combobox minimum size. 

The JCK tests mentioned in JBS seems fine with the fix. An automated test case is implemented to test in Nimbus and GTK LAF.

When swing tests are run with LAF set to Nimbus and GTK LAF, few test failures observed. 

Although CI jobs looks ok with tests run in their default set LAFs.

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Commit messages:
 - Combobox componnet rendering fix

Changes: https://git.openjdk.org/jdk/pull/11555/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11555&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8218474
  Stats: 182 lines in 5 files changed: 180 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/11555.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11555/head:pull/11555

PR: https://git.openjdk.org/jdk/pull/11555