Created the ObjectFinderWidget to handle object find.

Adjusted size of ParameterWidget (issue #214)

Removed unnacessary $$PWD var from .pro files
Minor fix on png export to avoid drawing of dirty areas.
Version upgrade 0.5.0_alpha -> 0.5.0_beta

Finished the object finder widget.

Renamed the namespace PgModeler to PgModelerNS

Minor improvements on ObjectsScene, ModelWidget, ModelObjectsWidget,
ObjectDepsRefsWidget

Changed the SQL validation usage on ModelValidationWidget
Updates on README.md and CHANGELOG.md.

Fix a bug that was crashing pgModeler when activating object finder
without a model loaded.