RFR: 6972078: Can not select single directory with GTKLookAndFeel

[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.

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Commit messages:
 - Whitespace error fix
 - Single directory selection fix

Changes: https://git.openjdk.org/jdk/pull/10866/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10866&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-6972078
  Stats: 122 lines in 2 files changed: 121 ins; 0 del; 1 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