Add confine-to-path node editing option

Adds the ability to slide a node along the existing line when pressing Alt

Also clears up the confusing situation between confine and confine_handles which previously depended on confine being true and thus the in effect key was actually Ctrl+Alt, not just Alt as previous stated.

The build_segment function was moved to node.cpp from path builder since it's a core bit of functionality for a node to be able to understand it's own geometry in terms of a Geom::PathVector.