Keep the position of the collapsed / expanded node

[kachkaev]

This is somewhat related to #5 and #10.
When a node contains too many children, clicking on it to expand or collapse causes a substantial shift of a tree. You have zoom out and search for the node you've just been looking at, which takes quite a lot of time. A solution could be in calculating x and y of a just-clicked node in two modes (when its children are hidden and visible) and then adjusting the global coordinate system accordingly.