#4 Redo/Undo should be atomic for complex ops

[BoC]

Many operations that include the manipulation of
multiple vertices/edges, such as opening a graph, doing
a complex DS layouting, etc, generate multiple undo
events. In another word, it might take a few dozen undo
to completely undo a graph layout operation. We should
make an undo/redo matches one USER operation, no matter
how many underlying graph manipulations are done.