fixed #139. Node positions were not set with respect to dragging offset
and zoom factor when moving subgraph.
Improved performance and simplified method of node and label position
updates;
Removed viewport borders. It is possible now to move nodes and labels
out of the window and later pan the viewport to it. See #125.
Resolves #125 by adding a parent list to all nodes and checking for the
children's and parents' position before updating position of clicked
node.