In GimpCurveView, when dragging an existing curve point, don't
immediately move the point to the cursor position uppon button
press, but rather move it relative to its current position as the
cursor moves.  This allows selecting a point without moving it, and
adjusting its position more easily.

Additionally, when the cursor hovers above a point, or when
dargging a point, have the coordinate indicator show the point's
position, rather than the cursor's.