#2 Possible to "lose" transition middle point

[Johan Garcia]

When modifying the appearance of a transition arc, it is currently possible to drag the middle point outside any of the borders. This will cause the point to be "lost" since it is not possible to grab it again.
Similarly, it is possible to move the state outside the lower and right borders (but not the upper or left...)

This should be fixed so that movements outside the borders are not possible.