#79 Nodes going to negative positions

[Rommel Novaes Carvalho]

The UShape that are not ellipsis (ex. decision and utility nodes) are moved a bit to the upper left direction once we click on the button "propagate" (or reinitialize beliefs) on the compilation pane. If we do that over and over again they eventually get outside the canvas (negative position) and we cannot get them back inside the canvas. This problem was some what addressed by not allowing negative values to the position of the nodes, but this should not be the final solution. I think the problem is related to the difference from the point of reference from the previous GUI and the new GUI. The previous one the position was related to the center of the node and now it is the upper left corner.