File chooser shortcuts panel

[DevCharly]

This PR adds a shortcuts panel to the left side of all file choosers (class JFileChooser):

On Windows, the content of the shortcuts panel is the same as in the Java Windows L&F.
On macOS and Linux the shortcuts panel is empty and hidden.

However you can customize the content. E.g.

UIManager.put( "FileChooser.shortcuts.filesFunction", (Function<File[], File[]>) files -> {
    ArrayList<File> list = new ArrayList<>( Arrays.asList( files ) );
    list.add( 0, new File( System.getProperty( "user.home" ) ) );
    return list.toArray( new File[list.size()] );
} );

UIManager.put( "FileChooser.shortcuts.displayNameFunction", (Function<File, String>) file -> {
    if( file.getAbsolutePath().equals( System.getProperty( "user.home" ) ) )
        return "Home";
    return null;
} );
UIManager.put( "FileChooser.shortcuts.iconFunction", (Function<File, Icon>) file -> {
    if( file.getAbsolutePath().equals( System.getProperty( "user.home" ) ) )
        return new FlatFileChooserHomeFolderIcon();
    return null;
} );

Downsides

The get the list of shortcut folders and to get the large icons, FlatLaf uses API that is not available in all Java versions.

• the Java API to get list of shortcut folders is available since Java 12; reflection is used for older Java versions
• the Java API to get large icons is available since Java 17; reflection is used for older Java versions

So for Java 9 to 16 you'll get following warning on stdout when a file chooser is opened.

WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by com.formdev.flatlaf.ui.FlatFileChooserUI$FlatShortcutsPanel (file:...) to method sun.awt.shell.ShellFolder.get(java.lang.String)
WARNING: Please consider reporting this to the maintainers of com.formdev.flatlaf.ui.FlatFileChooserUI$FlatShortcutsPanel
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release

If you use --illegal-access=deny, then the shortcuts panel will be empty (Java 9 to 11),
and small icons are scaled up (Java 9 to 18).

CC @Chrriis

[Chrriis]

There are several expectations with file choosers:

1. Faster navigation with shortcuts and perhaps some who want to customize these shortcuts.
2. People who want a more native file chooser in line with the look and feel.
3. People who want the native file chooser, independent from the look and feel.

I don't think FlatLaf should care about item 3 because there are other libraries that do it and it breaks the idea of a look and feel (I saw discussions on that topic lately, this is just my opinion).

This PR is more about 1 and 2:

• It does give people a way to customize the file chooser shortcuts.
• It feels more native because it has a navigation pane.

That being said, and correct me if I am wrong, it looks like the Windows 7 file chooser, or the AWT file chooser. All the file choosers that I can see on my Windows 10 have a tree on the left. My native file chooser are more like the second one from this (random) link:
https://stackoverflow.com/questions/39819319/windows-native-file-chooser-in-java

So, it is definitely an improvement, it solves item 1, but it is a partial answer to item 2. I understand the constraints so I know that it is not possible to support item 2 completely (like having a tree on the left, and editable crumb bar at the top).

I do welcome this change! Keep up the good work!