RFR: 6972078: Can not select single directory with GTKLookAndFeel [v2]

[Abhishek Kumar]

> While using a JFileChooser with the file selection mode FILES_AND_DIRECTORIES and multiSelection enabled, it is impossible to select a single directory in GTK LAF.
> 
> The condition check has been modified when multiselection is enabled and user has selected single directory.
> After the fix the behavior of JFileChooser is similar to other LAFs.
> 
> An automatic test case `test/jdk/com/sun/java/swing/plaf/gtk/TestFileChooserSingleDirectorySelection.java` is implemented and checked in CI pipeline. Link is attached in JBS.

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 fix
 - Merge
 - Whitespace error fix
 - Single directory selection fix

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/10866/files
  - new: https://git.openjdk.org/jdk/pull/10866/files/551a790b..c129d187

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10866&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10866&range=00-01

  Stats: 128073 lines in 785 files changed: 71055 ins; 27973 del; 29045 mod
  Patch: https://git.openjdk.org/jdk/pull/10866.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10866/head:pull/10866

PR: https://git.openjdk.org/jdk/pull/10866