Introduce a simple FileExplorer component for Drag-and-Drop Testing

Key features:
- ListView displays some files with a custom cell factory (FileListCell).
- Drag-and-drop functionality allows copying selected files.
- Files are created within the temp folder for testing purposes.

pdfsam-ui-components/src/test/java/org/pdfsam/ui/components/selection/multiple/MockFileExplorer.java

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+/*
+ * This file is part of the PDF Split And Merge source code
+ * Created on 27/nov/2013
+ * Copyright 2017 by Sober Lemur S.r.l. ([email protected]).
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+package org.pdfsam.ui.components.selection.multiple;
+
+import java.io.*;
+import java.net.*;
+import java.nio.file.*;
+import java.util.Collections;
+import javafx.scene.control.*;
+import javafx.scene.input.*;
+import javafx.scene.layout.VBox;
+
+class MockFileExplorer extends VBox {
+
+    MockFileExplorer(Path tempFolder) throws URISyntaxException, IOException {
+        var fileList = new ListView<File>();
+
+        fileList.setId("file-list");
+        fileList.setCellFactory(param -> new FileListCell());
+        fileList.setOnDragDetected(event -> onDragDetected(event, fileList));
+        fileList.setOnDragOver(this::onDragOver);
+
+        var pdfs = new File[]{
+            Files.createFile(tempFolder.resolve("1.pdf")).toFile(),
+            Files.createFile(tempFolder.resolve("2.pdf")).toFile()
+        };
+
+        fileList.getItems().addAll(pdfs);
+
+        getChildren().add(fileList);
+    }
+
+    private void onDragDetected(MouseEvent event, ListView<File> fileList) {
+        var selectedIndex = fileList.getSelectionModel().getSelectedIndex();
+
+        if (selectedIndex >= 0) {
+            var file = fileList.getItems().get(selectedIndex);
+            var dragboard = fileList.startDragAndDrop(TransferMode.COPY);
+            var content = new ClipboardContent();
+
+            content.put(DataFormat.FILES, Collections.singletonList(file));
+            dragboard.setContent(content);
+
+            event.consume();
+        }
+    }
+
+    private void onDragOver(DragEvent event) {
+        var dragboard = event.getDragboard();
+
+        if (dragboard.hasFiles()) {
+            event.acceptTransferModes(TransferMode.COPY);
+        }
+    }
+
+    private static class FileListCell extends ListCell<File> {
+
+        @Override
+        protected void updateItem(File item, boolean empty) {
+            super.updateItem(item, empty);
+
+            if (empty) {
+                setText(null);
+            } else {
+                setText(item.getName());
+            }
+        }
+    }
+
+}