Add Cancel button to Multiple Variable Editor

[MichaelBalzer]

The only option for closing the Multiple Variable Editor is with the Done button, or closing the window itself. In both cases any changes to the variables are applied, with no option to revert or cancel. There is an undo function, but it would be handy to be able cancel all changes in case sweeping mistakes were made across hundreds of variables (which is very easy to do if a column is selected).