add support for inserting Tab (\t) into the textual content

Signed-off-by: Florian Thienel <florian@thienel.org>
diff --git a/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/widget/swt/BoxWidget.java b/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/widget/swt/BoxWidget.java
index d5c34ca..6a9086f 100644
--- a/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/widget/swt/BoxWidget.java
+++ b/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/widget/swt/BoxWidget.java
@@ -761,6 +761,18 @@
 			}
 		});
 
+		// Tab
+		addKey(keyMap, SWT.TAB, SWT.TAB, SWT.NONE, new IVexWidgetHandler() {
+			@Override
+			public void execute(final ExecutionEvent event, final IDocumentEditor editor) throws ExecutionException {
+				try {
+					editor.insertChar('\t');
+				} catch (final DocumentValidationException e) {
+					throw new ExecutionException(e.getMessage(), e);
+				}
+			}
+		});
+
 		// {-, Shift, Ctrl, Shift+Ctrl} + Home/End
 		addKey(keyMap, CHAR_NONE, SWT.END, SWT.NONE, new IVexWidgetHandler() {
 			@Override