c++ QgsLayoutItem metadata classes can directly register
a function which creates a QgsLayoutViewRubberBand for the item
subclass.
Python code cannot utilise this shortcut (due to inaccessibility
of forward declared gui classes from core Python classes), so
there's a separate gui registry utility class added for registering
prototypes for rubber bands for already registered item types.
Copy the same model as QgsMapCanvas uses, with separate
classes for individual interaction tools instead of keeping
all the logic in the QGraphicsView subclass (as is done
with composer)
