IdealGraphVisualizer: work around a Swing/NetBeans tree UI problem that occurs when mouse events are handled while the tree model is changing, e.g. when using IGV while it still receives graphs from a compiler. BasicTreeUI attempts to get the boundaries for the node associated with the location on screen, but fails and tries to access a null Rectangle, causing a user-visible NullPointerException. By implementing equals() for GraphNode, nodes for graphs that are already in the tree are not removed and re-added when updating groups, and the exception can no longer be reproduced. This potentially also resolves some redraw issues.