Implemented an initial support for cached xml and sql codes. The code
generation time was hugely improved.
Huge improvement on model validation process. This improvement is
related mainly to reverse engineered models that, in theory, aren't
invalid but in previous versions pgModeler was raising validation errors
and the user needed to fix those problems even the model being
consistent on database server. Now pgModeler honors the imported
structure and in a few cases of db import there will be inconsistencies.

The objects rule, index and trigger will have the SQL/XML code generated
outside of table's definition due to validation module that sometimes
needs to swap id's of those objects and tables.

The swap object ids dialog was improved. Added a button to swap the
values of the fields.

The object selector will trigger the object selection if user click the
input field

Fixed the import of rule object on DatabaseImportHelper

The model objects widget gained a filter field that is capable to filter
objects by their name or by their id

Fixed a bug when displaying the cardinalities for fk relationships
(issue #512)
Code cleanup: removing unused and commented code.
Minor improvements on validation info display in ModelValidationWidget

Minor improvements on swap objects ids dialog.

Added an attribute called "proxy_object" on ValidationInfo class in
order to know the original object that generated the inconsistency.
Merge branch 'model-valid-timer' into db-import-improvements
Merge branch 'db-import-improvements' into develop