Introduced a new method BaseObject::getSignature() that returns by
default the object's name (formatted or not) and with the schema name
prepended (when available).

Code blocks where there's a special treatment to get object's name
filtering by object's type (specially function and operator) were
replaced by a single call to BaseObject::getSignature()

Fixed a crash when reverse engineering a model. The crash was due to
trying to handle an not existent graphical object for pg_catalog schema.
Model db diff feature impelementation checkpoint. Added a "Diff Preview"
tab in order to give user the final result of the whole process.