add a way to get chosen filter in SystemFileChooser

[Numerok]

On my own implementation i used windows HRESULT GetFileTypeIndex(IntByReference ppv) to get the selected filter index including .
But with SystemFileChooser when using ch.getFileFilter() i get the first filter and that breaks the logic of choosing the correct report output format for me.
Can it be added?

Originally posted by @Numerok in #988 (comment)

[nroduit]

I confirm this issue. SystemFileChooser does not currently behave like JFileChooser here.

In JFileChooser, getFileFilter() reflects the current filter selection in the UI. In SystemFileChooser, the value returned by getFileFilter() can remain the initial/programmatically set filter (or effectively the first filter), even if the user changes the filter dropdown in the native dialog. That’s why, in the following example, extension is always PNG and the JPEG branch is never reached:

SystemFileChooser fileChooser = new SystemFileChooser(targetDirectoryPath);
fileChooser.setFileSelectionMode(SystemFileChooser.FILES_ONLY);
fileChooser.setAcceptAllFileFilterUsed(false);
FileNameExtensionFilter filter =
  new FileNameExtensionFilter(Format.PNG.title, Format.PNG.extension);
fileChooser.addChoosableFileFilter(
  new FileNameExtensionFilter(Format.JP2.title, Format.JP2.extension));
fileChooser.addChoosableFileFilter(
  new FileNameExtensionFilter(Format.JPEG.title, Format.JPEG.extension));
fileChooser.addChoosableFileFilter(
  new FileNameExtensionFilter(Format.JPEG_XL.title, Format.JPEG_XL.extension));
fileChooser.addChoosableFileFilter(filter);
fileChooser.addChoosableFileFilter(
  new FileNameExtensionFilter(Format.TIFF.title, Format.TIFF.extension));
fileChooser.setFileFilter(filter);
filter = (FileNameExtensionFilter) fileChooser.getFileFilter();
String extension = filter == null ? Format.PNG.extension : filter.getExtensions()[0];
String extFile = "." + extension;
String filename =
    file.getName().endsWith(extFile) ? file.getPath() : file.getPath() + extFile;

File destinationFile = new File(filename);
if (Format.PNG.extension.equals(extension)) {
  ImageProcessor.writePNG(image.toMat(), destinationFile);
} else if (Format.JPEG.extension.equals(extension)) {
  // NEVER reached this line even when selecting the JPEG filter
} 

[DevCharly]

fixed in PR #1079