Add declarations for setPlaceholderString:/setPlaceholderAttributedString and getters to header and implementation to NSTextField.m

Source/NSTextField.m

@@ -120,6 +120,26 @@ static Class textFieldCellClass;
   return YES;
 }
 
+- (void) setPlaceholderString: (NSString *)string
+{
+  [_cell setPlaceholderString: string];
+}
+
+- (NSString *) placeholderString
+{
+  return [_cell placeholderString];
+}
+
+- (void) setPlaceholderAttributedString: (NSAttributedString *)string
+{
+  [_cell setPlaceholderAttributedString: string];
+}
+
+- (NSAttributedString *) placeholderAttributedString
+{
+  return [_cell placeholderAttributedString];
+}
+
 /** <p>Returns whether the NSTextField is editable. By default a NSTextField
     is not editable.</p>
     <p>See Also: -setEditable: [NSCell-isEditable]</p>