Added beginnings of tabbed documents support.

Reviewed by me.

AppKit/CPWindow/CPWindow.j

@@ -248,6 +248,7 @@ var CPWindowSaveImage       = nil,
     BOOL                                _hasShadow;
     BOOL                                _isMovableByWindowBackground;
 
+    BOOL                                _supportsMultipleDocuments;
     BOOL                                _isDocumentEdited;
     BOOL                                _isDocumentSaving;
 
@@ -441,6 +442,19 @@ CPTexturedBackgroundWindowMask
     return _CPBorderlessBridgeWindowView;
 }
 
+- (void)setSupportsMultipleDocuments:(BOOL)shouldSupportMultipleDocuments
+{
+    shouldSupportMultipleDocuments = !!shouldSupportMultipleDocuments;
+
+    // FIXME: throw exception if window controller already has multiple documents and shouldSupportMultipleDocuments === NO
+    _supportsMultipleDocuments = shouldSupportMultipleDocuments;
+}
+
+- (BOOL)supportsMultipleDocuments
+{
+    return _supportsMultipleDocuments;
+}
+
 - (void)awakeFromCib
 {
     if (_initialFirstResponder)