RFR: 4912623: GTK L&F: Folder list of the JFileChooser is allowing multiple selection unlike native [v3]

[Abhishek Kumar]

> It is not possible to select all files on press of Ctrl+A in JFileChooser for GTK LAF where as all folders can be selected. 
> In native side on press of Ctrl+A, able to select all files as well as folders.
> 
> Proposed fix is to enable `multiselection `property by default for JFileChooser in GTK LAF to mimic native side behavior.
> 
> An automated test case is implemented and tested in CI. CI job links are 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:

 - Subtest added for multiselection disabled
 - Merge
 - Update for folder list and test review comment update
 - Select all files on CTrl+A fix

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/11627/files
  - new: https://git.openjdk.org/jdk/pull/11627/files/2a8f892d..52fa7cba

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11627&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11627&range=01-02

  Stats: 17027 lines in 441 files changed: 11687 ins; 3883 del; 1457 mod
  Patch: https://git.openjdk.org/jdk/pull/11627.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11627/head:pull/11627

PR: https://git.openjdk.org/jdk/pull/11627