Added the method BaseObjectWidget::setRequiredField() that highlight the
specified widget as a required field on form.

Added the system_obj property to BaseObject that indicates if the object
is a PostgreSQL reserved object (e.g, schema public, languages SQL, C).

Minor fixes on syntax highlighter. Now when a word ground does'nt has a
background defined it will be transparent bg color.

Models now are correclty auto saved when are modified (issue #188)

Fixed operator's signature generation.