(gui) Externalize code in 2 new methods (InstancePattern)

When updating refinement, existing connections are saved / restored
in specific methods
(gui) Allow refinement on the same entity as before

The case will be managed in a better way in next commits
(gui) Save/restore connections only when needed

Also remove the second signature of InstancePattern#setInstanceRefinement
(gui) Update doc
(gui) Do not check network validity each time it is displayed

Now, validity checks are trigered only when the diagram is
initialized for the first time.
(gui) Prevent refinement on the same entity as before

This is the better mentionned in a90f6258133