(gui) Fix an exception when selecting an instance by drawing an area
(gui) Fix a tricky bug introduced in ce7fafabd1e173

Removing the line 196 from UngroupNetworkFeature was a bad idea. This
cause serialization exceptions in Network after applying the feature.

Now, cleaning connections before deleting the old instance is not necessary
anymore. The 'delete' method in InstancePattern clean everything connected
to it (business object, connections and their own business objects). Since
a Network object "contains" (in EMF world) each connection, deleting a connection
remove it from its containing network.
(gui) Simplify and document XdfUtil.getAddConnectionContext()