[BISERVER-11217] - Added creatable flag, omit datasource types from crea...

...te new list with creatable flag = false

Some additional notes: We originally only had the isEditable/Importable/() etc. methods for instances of DatasourceInfo and not the types themselves. I added those methods to the interface for the type because we would otherwise have a hard time determining those properties for types unless we had instances of those types present. This was not really an issue until this implementation of the creatable flag.