Fixed a mistake in VectorialTableModel.

Row number was not being translated from GUI
to model.
Translation from GUI to model simplified in a menu

One of the contextual menus had a complex method
for translating GUI row number to model for no
reason.
FormFactory transformed in a true static factory.

Now FormFactory has a default instance that should
be created by subclasses, and which we can transpa-
rently access in a static way.
If there is no instance, nothing's done.
Added more static methods to the factory.
NN rel handlers check that table/layer are loaded.

They call the FormFactory in order to check with
the registered factory.
Added a new table contextual menu for layer forms.
Added basic relationship table handlers.

We have a Base one which is extended by a vectorial
one and an alphanumeric one.
NN rel base table handler extends the simple one
Factory methods for creating forms by layername.
Contextual menus heavily refactored.

We redesigned their hierarchy so we have one
branch for alphanumeric ones and another
for vectorial ones.
Alphanumeric table handler must fill foreign key.

The subform must be filled with the foreign key
so we can create new elements.
Vectorial editable NN rel context menu refactored.

Now it fits better into the hierarchy and it creates
by itself the Singleton form using the factory.