internal change - network creator GUI split
internal changes:
- x_change/y_change removed, MouseListener is now added to the panel instead of the container
- some repetitions extracted to Util
- some javadocs added
NETWORK CREATOR INTERFACE UPDATE:
NEW: while connecting nodes, if the mouse button is released where there is no node, a new one is created and connected
CHANGE: node selection is now updated when mouse button is released (after connecting/disconnecting nodes)
BUGFIX: in help there was no information how to use deletion with keeping connections - it has been fixed

INTERNAL:
CHANGE: node selection is now remembered as integer (node id) rather than Node