Introduce networking mode

[Tom Roshko]

In many infrastructure diagrams, network resources are very important, in others networking is irrelevant.

For that reason, it is suggested to introduce a diagram networking mode which the user can tweak. This can serve multiple types of diagrams focusing and un-focusing the networking resources.

Suggested values/modes are:

NORMAL - As currently diagramed by CDK-Dia.
OMIT - no networking resources diagramed
FULLY_EXPANDED - all networking resources diagramed. this should override collapsing of resources which causes many resources to be omitted.

[Sholto Maud]

Perhaps export multiple diagrams as different views rather than expanding/collapsing?