Changes to make containers behave like a first-class object.

Changes to make containers behave like a first-class object.

Also, added cut/copy/paste functionality, and ability to save layer
information with object data.  This change bumps the drawing version to
1.2; this required some infrastructure changes to the way layers are
represented and handled.  Also preliminary addition of Trim, Triangulate,
and Parallel tools, and support for snapping to line midpoints.