Support CPThemeStateEditable for text fields.

AppKit/CPTextField.j

@@ -308,6 +308,11 @@ CPTextFieldStatePlaceholder = CPThemeState("placeholder");
     if (shouldBeEditable)
         _isSelectable = YES;
 
+    if (_isEditable)
+        [self setThemeState:CPThemeStateEditable];
+    else
+        [self unsetThemeState:CPThemeStateEditable];
+
     // We only allow first responder status if the field is editable and enabled.
     if (!shouldBeEditable && [[self window] firstResponder] === self)
         [[self window] makeFirstResponder:nil];