Database Providers; GetSchema column information updated to better support different types of columns

Changes have been made to handle VARCHAR versus NVARCHAR columns in SQL Server. The DataSubType enum for columns is now set to "Unicode" for NVARCHAR columns.

Note, in Access databases Text/ Memo columns are always Unicode.

The Database Documenter now shows columns for SQL Server as VARCHAR and NVARCHAR, as well as "Memo" columns being VARCHAR(Max) or NVARCHAR(Max). Access database show as TEXT and MEMO.

Note, ISDatabaseColumn.DataSubType has changed from an Integer to iseColumnSubType enum (which under the skin it always was). This may cause code using this to break.