Eliminate compiler warnings: 1) adjust constructor initial list order;
2) suppress unused variable warnings with Q_UNUSED().
Change global MAX_VALUE to a const static private member of class Node
Support auto-adjustable edge
Edge enhancements:
1) Draw a directed edge by Shift + left clicking and moving.
2) Draw a edge indicator (QGraphicsLineItem) when creating an edge by
moving mouse.
3) Do not display an edge if its 2 terminal nodes collide with each
other.