ModelValidationWidget finished. Pending code comments.

Minor fixes on database model code generation.
ModelValidationWidget code comment done.
Simplified the method DatabaseModel::loadModel(). The method does not
try to recreate a object when some dependency was not found instead
errors are raised.
Minor improvements when moving objects over the scene. Relationship
points are moved too if the parent relationship is being moved together
to other objects.

Fixed relationship loading which was not remembering label positions.
Added a key navigation on model widget. Alt + <left|right> keys switch
between graphical objects.

Added a warning message on mainwindow when the user try to save a
invalidated model.

pgModeler now aborts app closing when the user asks for save modified
models.