Enable Spotfire expression language for in-database connections

When working with external data one cannot use the standard Spotfire expressions but has to change to the dialect of the given data source (e.g. when working with an Impala connection a Left([Column_A],1) for imported data has to be changed to Substring([Column_A],1,1) if the same data is kept external. This creates two problems:

1. Changing data from imported to external or the other way round will break any analysis with calculated columns.
2. The average user cannot be bothered with learning the various dialects such that the self-service concept fails with external data
   1. because people just don't know all the dialects
   2. because there is no help on the syntax for external data (instead only See the official Cloudera Impala documentation for a description of the Abs function. is displayed)

A simple mapping of Spotfire expressions to the appropriate syntax of various data sources would enable both switching between external and imported data and self-service use by non-IT experts. Spotfire expressions not supported by a given data source could simply be hidden or greyed out.