app: in GimpCurveView, use relative motion when dragging point
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.
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