#7 DSI sometimes do not ask user to save after graph changed

[BoC]

Many basic element manipulation in DSI are done without
the intervention of our code (e.g handled directly and
automatically by JGraph), such as dragging and dropping
a vertex. The current code that prompts user to save
changes do not know of these kind of operations. So if
someone open an existing document and moves a simple
vertex by drag and drop, DSI will not prompt the user
to save when he exits.