CPWindow alpha value support.

AppKit/CPWindow/CPWindow.j

@@ -883,6 +883,23 @@ CPTexturedBackgroundWindowMask
     return _contentView;
 }
 
+/*!
+    Applies an alpha value to the window.
+    @param aValue the alpha value to apply
+*/
+- (void)setAlphaValue:(float)aValue
+{
+    [_windowView setAlphaValue:aValue];
+}
+
+/*!
+    Returns the alpha value of the window.
+*/
+- (float)alphaValue
+{
+    return [_windowView alphaValue];
+}
+
 /*!
     Sets the window's background color.
     @param aColor the new color for the background