Editing forms aren't singletons anymore. They will be used on demand by
ModelWidget class. This could improve the software startup. There some
adjustments still pending.
Finished the removal of almost all singletons present on the
application. The only one left is the ConfigurationForm that must be
accessible to other classes in order to them read the current
configurations.
Removed commented and unnused code.
Messagebox objects are now used on demand and not more as class
attributes.
Minor change on object's id interval generation.

Added a method setCodesInvalidated() on DatabaseModel to force the code
invalidation of all objects

Minor fix on sample models.

Minor adjust on model export process to force the code cache
invalidation when using the unique name generation option.
SQLAppendWidget renamed to CustomSQLWidget

Minor improvement on model validation. Now the ddl executed for any
object is shown on the output field.
Minor fix on ModelExportForm

The export, import and fix dialogs where moved from MainWindow attribute
list to local variables, respectively, in methods exportModel and
importDatabase and fixModel.
Methods parseArrayValues and parseDefaultValues moved from
DatabaseImportHelper to Catalog

DataManipulationForm implementation checkpoint.
Added the method ResultSet::getColumnTypeId to return the id of column's
data type

Added the method Catalog::getFilter to return the current filter state

Minor enhacement on SQLToolWidget::fillResultsTable that include the
columns' data type as header tooltips

DataManipulationForm implementation checkpoint
Minor fix on SQLToolWidget to disable controls in some situations.

Several icon changes.

DataManipulationForm implementation checkpoint
Fixed a crash whenever the user canceled the changes on pgModeler
settings and tried to connect to a server using the sql tool.
DataManipulationForm implementation checkpoint. Pending the insert,
delete and update through the results grid.
Merge branch 'singletons-removal' into data-manipulation