RFR: 8218474: JComboBox display issue with GTKLookAndFeel [v2]

[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. The text color check is to ensure the current selected item is visible.
> 
> 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.
> 
> `open/test/jdk/javax/swing/JComboBox/TestComboBoxHeight.java` test passes if run in GTK LAF with current fix.

Abhishek Kumar has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - Review comment update
 - Merge
 - Merge
 - Combobox componnet rendering fix

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/11555/files
  - new: https://git.openjdk.org/jdk/pull/11555/files/b0db194b..ea53f8ae

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11555&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11555&range=00-01

  Stats: 159620 lines in 2761 files changed: 77674 ins; 58356 del; 23590 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