Allow pasting text into SwingFileWidget

This restores the ability to paste text into the JTextField of SwingFileWidget. Before this change, the custom TransferHandler did not accept String values.
Now we try to get a File value first, and fall back to getting the String value of the current transfer operation.

I also tried to restore the full default Copy/Cut/Paste functionality by calling super.importData(support), but couldn't get it to work. So let's stay with some minimal Paste functionality at least.

Author: imagejan

src/main/java/org/scijava/ui/swing/widget/SwingFileWidget.java

@@ -288,6 +288,24 @@ public static List<File> getFiles(
 		}
 	}
 
+	/**
+	 * Gets the String content of the current transfer support
+	 * 
+	 * @param support
+	 *            The paste (or drag and drop) operation from which text should
+	 *            be extracted.
+	 * 
+	 * @return The pasted (or dropped) text
+	 */
+	public static String getText(final TransferHandler.TransferSupport support) {
+		try {
+			return (String) support.getTransferable().getTransferData(DataFlavor.stringFlavor);
+		} catch (UnsupportedFlavorException | IOException exc) {
+			return "";
+		}
+
+	}
+
 	/**
 	 * Filters the given list of files according to the specified
 	 * {@link FileFilter}.
@@ -348,7 +366,15 @@ public boolean canImport(final TransferHandler.TransferSupport support) {
 		@Override
 		public boolean importData(TransferHandler.TransferSupport support) {
 			final List<File> files = getFiles(support);
-			if (files == null || files.size() != 1) return false;
+			if (files == null) {
+				String text = getText(support);
+				if (text.equals(""))
+					return false;
+				// TODO check if text matches filter/style
+				((JTextField) support.getComponent()).setText(text);
+				return true;
+			}
+			if (files.size() != 1) return false;
 
 			final File file = files.get(0);
 			((JTextField) support.getComponent()).setText(file.getAbsolutePath());