stateless QueryModels don't support dates

It's currently impossible to use date/time in a stateless query model. The problem is that the QM can't guess whether the user wants to produce
a DATE or TIMESTAMP where clause part.

The solution would be that querymodel should accept instances of org.nuxeo.ecm.core.query.sql.Literal. Then client code (typically in a Seam Component) can use DateLiteral or TimestampLiteral, or whatever.