Widget/Operator instantiation in wiring using endpoint type

[svavassori]

It would ease a lot the composition of wiring if, selecting/right-click a widget/operator endpoint, the system displays a list of compatible widgets/operator and clicking on it will instantiate that widget/operator and wire it to the previously selected endpoint.

In this way the list of widgets/operators to search into is reduced to only the compatible ones, improving and speeding up the composition of wiring, especially if there are a lot widgets/operators with few compatible relationships with others.

[svavassori]

see as reference #104