RFR: 4912623: GTK L&F: Folder list of the JFileChooser is allowing multiple selection unlike native [v2]

[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 incrementally with one additional commit since the last revision:

  Update for folder list and test review comment update

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11627/files
  - new: https://git.openjdk.org/jdk/pull/11627/files/4c12d5ba..2a8f892d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11627&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11627&range=00-01

  Stats: 12 lines in 2 files changed: 10 ins; 0 del; 2 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