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

[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.

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Commit messages:
 - Select all files on CTrl+A fix

Changes: https://git.openjdk.org/jdk/pull/11627/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11627&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-4912623
  Stats: 170 lines in 2 files changed: 170 ins; 0 del; 0 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