RFR: 6972078: Can not select single directory with GTKLookAndFeel [v6]

[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 incrementally with one additional commit since the last revision:

  Review comment fix

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/10866/files
  - new: https://git.openjdk.org/jdk/pull/10866/files/0b7d1a6e..5c5c564c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10866&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10866&range=04-05

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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