store: Added sdb_store_get_field().

This function may be used to retrieve the value of queryable fields of objects
in the store.

For now, all fields except the backend(s) are supported.
store_expr: Added support to include field values in an expression.

Added sdb_store_expr_fieldvalue() to create such an expression object and pass
a stored object to sdb_store_expr_eval() to be able to extract the field value
when evaluation the expression.
frontend/grammar: Added support for field names in expressions.