AppKit: Disable Undo and Redo when a text view does not allow undo

A text view handled undo: and redo: and registered its changes even when allowsUndo was NO, so Edit > Undo stayed enabled and did nothing.

Behaviour now follows canonical Cocoa: a text view that does not allow undo registers nothing and passes undo: and redo: on to the window, and Undo and Redo are enabled only when there is a change to undo or redo.

* CPTextView/CPTextView.j: Register changes only when undo is allowed, stop responding to undo: and redo: when it is not, and validate the Undo and Redo items against the undo manager.

* CPWindow/CPWindow.j: Validate the Undo and Redo items against the window's undo manager.

* Tests/AppKit/CPTextViewTest.j: Add tests for responding to undo:, registering changes and validating the Undo item.

* Tests/Manual/TextEditor/DocumentWindowController.j: Add toggleAllowsUndo: and validateMenuItem:, so that undo can be turned on and off for the active document.

* Tests/Manual/TextEditor/Resources/MainMenu.xib, Resources/MainMenu.cib: Add Edit > Allow Undo.

* Tests/Manual/TextEditor/WORKLIST.md: Remove the entry for this gap, record Allow Undo as tested, and add the key equivalent behaviour of disabled menu items for discussion.

Tests/AppKit/CPTextViewTest.j

@@ -136,6 +136,56 @@
     [delegateSpy verifyThatAllExpectationsHaveBeenMet];
 }
 
+- (void)testTextViewRespondsToUndoOnlyWhenUndoIsAllowed
+{
+    [textView setAllowsUndo:YES];
+
+    [self assertTrue:[textView respondsToSelector:@selector(undo:)]];
+    [self assertTrue:[textView respondsToSelector:@selector(redo:)]];
+
+    [textView setAllowsUndo:NO];
+
+    [self assertFalse:[textView respondsToSelector:@selector(undo:)] message:@"undo: should continue to the window"];
+    [self assertFalse:[textView respondsToSelector:@selector(redo:)] message:@"redo: should continue to the window"];
+}
+
+- (void)testTextViewRegistersUndoOnlyWhenUndoIsAllowed
+{
+    var undoManager = [theWindow undoManager];
+
+    [self assertTrue:[theWindow makeFirstResponder:textView]];
+
+    [textView setAllowsUndo:NO];
+    [undoManager removeAllActions];
+    [textView insertText:@"Not recorded"];
+
+    [self assertFalse:[undoManager canUndo] message:@"a text view that does not allow undo should register nothing"];
+
+    [textView setAllowsUndo:YES];
+    [textView insertText:@"Recorded"];
+
+    [self assertTrue:[undoManager canUndo] message:@"a text view that allows undo should register its change"];
+}
+
+- (void)testUndoItemIsEnabledOnlyWhenThereIsAChangeToUndo
+{
+    var undoManager = [theWindow undoManager],
+        undoItem = [[CPMenuItem alloc] initWithTitle:@"Undo" action:@selector(undo:) keyEquivalent:@""],
+        redoItem = [[CPMenuItem alloc] initWithTitle:@"Redo" action:@selector(redo:) keyEquivalent:@""];
+
+    [self assertTrue:[theWindow makeFirstResponder:textView]];
+    [undoManager removeAllActions];
+
+    [self assertFalse:[textView validateUserInterfaceItem:undoItem]];
+    [self assertFalse:[theWindow validateUserInterfaceItem:undoItem]];
+    [self assertFalse:[textView validateUserInterfaceItem:redoItem]];
+
+    [textView insertText:@"Recorded"];
+
+    [self assertTrue:[textView validateUserInterfaceItem:undoItem]];
+    [self assertTrue:[theWindow validateUserInterfaceItem:undoItem]];
+}
+
 @end
 
 @implementation CPTextViewTest (CPTextViewTestDelegate)