Fix drag position after alternate button press

When a button is released and the edit state is not "dragging", reset
the drag delta to zero.

There was a problem with the spot removal spots being dragged. If a spot
is deleted while being dragged, dragging another spot will cause a jump.

rtgui/spot.cc

@@ -667,6 +667,7 @@ bool Spot::button3Pressed (int modifierKey)
         return true;
     } else if (! (modifierKey & (GDK_SHIFT_MASK | GDK_SHIFT_MASK))) {
         EditSubscriber::action = EditSubscriber::Action::PICKING;
+        return true;
     }
 
     return false;