Changed node layout
This time the SceneRect of QGraphicsScene will be used for the nodes placement instead of the users viewport.
At every 100 node contained in the graph the sceneRect will be scaled before the placement, so that space between the nodes is enough to read.
After the placement of all nodes in the graph, the rectangle of the viewer (QGraphicsView) will be resized to fit the graph with an additional offset for better navigation.
Closes #24 (closed)