[Dev] commitAfterDispatch attribute on BlockEvent

[John Anderson]

I just added an attribute to a BlockEvent, commitAfterDispatch, which 
commits after the event is processed. Doing the commit on the command 
allowed me to eliminate a bunch of unnecessary commits, and allows us to 
eventually use commits as the unit of undo. So for commands that you 
would like to be undoable, e.g. like cut copy paste, etc., set 
commitAfterDispatch to True.

John