Delete maintains connections

Deleting nodes attempts to rebuild the network to act as if the deleted nodes had been disabled. This required adding an official Enable/Disable behaviour for nodes, which provides a means to query if a node is disable-able, it's current enabled state, and a correspondence between it's input plug and output plugs. By providing this correspondence, the node is promising that the input will pass-through to the output in some meaningful way when the node is disabled.

Fixes Issue #95.