Guided Decision Table Editor: Add support for caching enumeration lookups

Currently the Guided Decision Table Editor calls to the server to evaluate enumerations defined to use a server-side helper class. Each cell using such an enumeration makes individual calls. This can lead to a very chatty behaviour and is a factor making certain decision tables tardy in response to user interactions. Furthermore since we now use a HTML Canvas for the table callbacks setting the Text of a cell complete after the Canvas has been rendered and hence do not update the view (without a redraw). Caching enumerations also prevents the need to perform an asynchronous call and the cell content can be set synchronously when the table is rendered.