Fixed the "ignore duplicity" bug for columns when exporting model (#271)

Added an option to generate columns and constraints via ALTER command
instead of being generated inside table's declaration (experimental).
Note: exporting to DBMS using "ignore duplicity" implies enabling the
ALTER command generation (user cannot control this).

Minor improvements on model export class. Options "Ignore duplicity" and
"simulate export" cannot be used together anymore since pgModeler isn't
able to undo columns additions created via ALTER command. This can be
changed in the future.

Catalog class implementation checkpoint.