Construct a new ModeModify with the given parent, and set the
Anchor point to a default location (the _anchor's proper position
will be determioned on mouse down).
When the user drags the mouse two things can happen:
(1) if the user is dragging the body of one or more Figs then
they are all moved around the drawing area, or
(2) if the user started dragging on a handle of one Fig then the user
can drag the handle around the drawing area and the Fig reacts to that.