Disable Java GUI components

Adding `-Djava.awt.headless=true` apparently prevents the annoying, laggy
GUI popups when launched ZXing CLR on MacOS or Windows.
Found at https://stackoverflow.com/a/60828709/20789

Author: dlenski

zxing/__init__.py

@@ -112,7 +112,7 @@ def decode(self, filenames, try_harder=False, possible_formats=None, pure_barcod
                 fn = fn_or_im
             file_uris.append(pathlib.Path(fn).absolute().as_uri())
 
-        cmd = [self.java, '-cp', self.classpath, self.cls] + file_uris
+        cmd = [self.java, '-Djava.awt.headless=true', '-cp', self.classpath, self.cls] + file_uris
         if self.zxing_version_info and self.zxing_version_info >= (3, 5, 3):
             # The --raw option was added in 3.5.0, but broken for certain barcode types (PDF_417 and maybe others) until 3.5.3
             # See https://github.com/zxing/zxing/issues/1682 and https://github.com/zxing/zxing/issues/1683