Further progress on Polylines: Polylines can be selected and moved.

This required a bit of shuffling under the hood to support this, but the
short version is that now Polylines will properly respond to mouse
movement near their contours and control points.  They still can't be
modified or created as of yet, but that's coming.  :-)  Also, slight
change to Circle and Arc handling: now clicking on their contours will
translate them instead of changing their radii.  Holding SHIFT will
allow changing their radii.